<option>-XRankNTypes</option>,
<option>-XImpredicativeTypes</option>,
<option>-XTypeOperators</option>,
- <option>-XRecursiveDo</option>,
+ <option>-XDoRec</option>,
<option>-XParallelListComp</option>,
<option>-XEmptyDataDecls</option>,
<option>-XKindSignatures</option>,
<title>The recursive do-notation
</title>
-<para> The recursive do-notation (also known as mdo-notation) is implemented as described in
-<ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>,
-by Levent Erkok, John Launchbury,
-Haskell Workshop 2002, pages: 29-37. Pittsburgh, Pennsylvania.
-This paper is essential reading for anyone making non-trivial use of mdo-notation,
-and we do not repeat it here.
-</para>
<para>
-The do-notation of Haskell does not allow <emphasis>recursive bindings</emphasis>,
+The do-notation of Haskell 98 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.
+the do-notation. The <option>-XDoRec</option> flag provides the necessary syntactic support.
</para>
<para>
-Here is a simple (yet contrived) example:
-</para>
+Here is a simple (albeit contrived) example:
<programlisting>
+{-# LANGUAGE DoRec #-}
import Control.Monad.Fix
-justOnes = mdo xs <- Just (1:xs)
- return xs
+justOnes = do { rec { xs <- Just (1:xs) }
+ ; return (map negate xs) }
</programlisting>
+The <literal>rec</literal>
+As you can guess <literal>justOnes</literal> will evaluate to <literal>Just [-1,-1,-1,...</literal>.
+</para>
<para>
-As you can guess <literal>justOnes</literal> will evaluate to <literal>Just [1,1,1,...</literal>.
+The background and motivation for recusrive do-notation is described in
+<ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>,
+by Levent Erkok, John Launchbury,
+Haskell Workshop 2002, pages: 29-37. Pittsburgh, Pennsylvania.
+This paper is essential reading for anyone making non-trivial use of mdo-notation,
+and we do not repeat it here. However, note that GHC uses a different syntax than the one
+in the paper.
</para>
+<sect3>
+<title>Details of recursive do-notation</title>
+<para>
+The recursive do-notation is enabled with the flag <option>-XDoRec</option> or, equivalently,
+the LANGUAGE pragma <option>DoRec</option>. It introduces the single new keyword "<literal>rec</literal>",
+which wraps a mutually-recusrive group of monadic statements,
+producing a single statement. Similar to a <literal>let</literal>
+statement, the variables bound in the <literal>rec</literal> are
+visible throughout the <literal>rec</literal> group, and below it.
+</para>
<para>
The Control.Monad.Fix library introduces the <literal>MonadFix</literal> class. Its definition is:
</para>
dictates how the required recursion operation should be performed. For example,
<literal>justOnes</literal> desugars as follows:
<programlisting>
-justOnes = mfix (\xs' -> do { xs <- Just (1:xs'); return xs }
+justOnes = do { xs <- mfix (\xs' -> do { xs <- Just (1:xs'); return xs })
+ ; return (map negate xs) }
</programlisting>
-For full details of the way in which mdo is typechecked and desugared, see
-the paper <ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>.
-In particular, GHC implements the segmentation technique described in Section 3.2 of the paper.
-</para>
-<para>
-If recursive bindings are required for a monad,
-then that monad must be declared an instance of the <literal>MonadFix</literal> class.
-The following instances of <literal>MonadFix</literal> are automatically provided: List, Maybe, IO.
-Furthermore, the Control.Monad.ST and Control.Monad.ST.Lazy modules provide the instances of the MonadFix class
-for Haskell's internal state monad (strict and lazy, respectively).
+In general, a <literal>rec</literal> statment <literal>rec <replaceable>ss</replaceable></literal>
+is desugared to the statement
+<programlisting>
+ <replaceable>vs</replaceable> <- mfix (\~<replaceable>vs</replaceable> -> do { <replaceable>ss</replaceable>
+ ; return <replaceable>vs</replaceable> })
+</programlisting>
+where <replaceable>vs</replaceable> is a tuple of the varaibles bound by <replaceable>ss</replaceable>.
+Moreover, the original <literal>rec</literal> typechecks exactly
+when the above desugared version would do so. (For example, this means that
+the variables <replaceable>vs</replaceable> are all monomorphic in the statements
+following the <literal>rec</literal>, because they are bound by a lambda.)
</para>
<para>
-Here are some important points in using the recursive-do notation:
+Here are some other 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>).
+It is enabled with the flag <literal>-XDoRec</literal>, which is in turn implied by
+<literal>-fglasgow-exts</literal>.
</para></listitem>
<listitem><para>
-It is enabled with the flag <literal>-XRecursiveDo</literal>, which is in turn implied by
-<literal>-fglasgow-exts</literal>.
+If recursive bindings are required for a monad,
+then that monad must be declared an instance of the <literal>MonadFix</literal> class.
+The following instances of <literal>MonadFix</literal> are automatically provided: List, Maybe, IO.
+Furthermore, the Control.Monad.ST and Control.Monad.ST.Lazy modules provide the instances of the MonadFix class
+for Haskell's internal state monad (strict and lazy, respectively).
</para></listitem>
<listitem><para>
</para></listitem>
<listitem><para>
-Variables bound by a <literal>let</literal> statement in an <literal>mdo</literal>
-are monomorphic in the <literal>mdo</literal> (Section 3.1 of the paper). However
-GHC breaks the <literal>mdo</literal> into segments to enhance polymorphism,
-and improve termination (Section 3.2 of the paper).
+Similar to let-bindings, GHC implements the segmentation technique described in Section 3.2 of
+<ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>,
+to break up a single <literal>rec</literal> statement into a sequenc e of statements with
+<literal>rec</literal> groups of minimal size. This
+improves polymorphism, and reduces the size of the recursive "knot".
</para></listitem>
</itemizedlist>
</para>
+</sect3>
+<sect3> <title Mdo-notation (deprecated) </title>
+
+<para> GHC used to support the flag <option>-XREecursiveDo</option>,
+which enabled the keyword <literal>mdo</literal>, precisely as described in
+<ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>,
+but this is now deprecated. Instead of <literal>mdo { Q; e }</literal>, write
+<literal>do { rec Q; e }</literal>.
+</para>
<para>
Historical note: The old implementation of the mdo-notation (and most
of the existing documents) used the name
<literal>MonadRec</literal> for the class and the corresponding library.
This name is not supported by GHC.
</para>
+</sect3>
</sect2>
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