[project @ 2002-10-01 15:59:03 by erkok]

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

diff --git a/ghc/docs/users_guide/glasgow_exts.sgml b/ghc/docs/users_guide/glasgow_exts.sgml
index 56d2221..d4a39be 100644 (file)
--- a/ghc/docs/users_guide/glasgow_exts.sgml
+++ b/ghc/docs/users_guide/glasgow_exts.sgml
@@ -973,8 +973,31 @@ example, we define the <literal>min</literal> function by binding
   min  = let ?cmp = (<=) in least
 </programlisting>
 <para>
-Note the following additional constraints:
+Note the following points:
 <itemizedlist>
+<listitem><para>
+You may not mix implicit-parameter bindings with ordinary bindings in a 
+single <literal>let</literal>
+expression; use two nested <literal>let</literal>s instead.
+</para></listitem>
+
+<listitem><para>
+You may put multiple implicit-parameter bindings in a
+single <literal>let</literal> expression; they are <emphasis>not</emphasis> treated
+as a mutually recursive group (as ordinary <literal>let</literal> bindings are).
+Instead they are treated as a non-recursive group, each scoping over the bindings that
+follow.  For example, consider:
+<programlisting>
+  f y = let { ?x = y; ?x = ?x+1 } in ?x
+</programlisting>
+This function adds one to its argument.
+</para></listitem>
+
+<listitem><para>
+You may not have an implicit-parameter binding in a <literal>where</literal> clause,
+only in a <literal>let</literal> binding.
+</para></listitem>
+
 <listitem>
 <para> You can't have an implicit parameter in the context of a class or instance
 declaration.  For example, both these declarations are illegal:
@@ -2605,6 +2628,91 @@ qualifier list has just one element, a boolean expression.
 </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">
@@ -4001,6 +4109,7 @@ classes usually have one "main" parameter for which deriving new
 instances is most interesting.
 </para>
 </sect2>
+
 </sect1>