[added MonadPlus
Jeremy Shaw <jeremy@seereason.com>**20100519173931
 Ignore-this: 5c0b6491d003a7cff508bd59d41d40a1
] hunk ./Makefile 13
-RouteFilters.html: Dir.lhs Dir2.lhs Dirs.lhs
+RouteFilters.html: MonadPlus.lhs Dir.lhs Dir2.lhs Dirs.lhs
addfile ./MonadPlus.lhs
hunk ./MonadPlus.lhs 1
+<!--
+
+> {-# LANGUAGE CPP #-}
+
+-->
+
+<h2>Choosing between multiple <code>ServerPartTs</code></h2>
+
+<p>In the first example, we had only one <code>ServerPartT</code>. All
+<code>Request</code>s were handled by the same part and returned the
+same <code>Response</code>.
+
+<p>In general, our applications will have many
+<code>ServerPartT</code>s. We combine them into a single top-level
+<code>ServerPartT</code> by using <code>MonadPlus</code>.
+
+<p>In the following example we combine three <code>ServerPartT</code>s
+together using the <code>MonadPlus</code> function.
+
+#ifdef HsColour
+> msum :: (MonadPlus m) => [m a] -> m a
+#endif
+
+> module Main where
+>
+> import Control.Monad
+> import Happstack.Server (nullConf, simpleHTTP, ok, dir)
+>
+> main :: IO ()
+> main = simpleHTTP nullConf $ msum [ mzero
+>                                   , ok "Hello, World!"
+>                                   , ok "Unreachable ServerPartT"
+>                                   ]
+
+<p class="source-code">[Source code for the app is <a href="MonadPlus.hs">here.</a>]</p>
+
+<p>The behaviour of <code>MonadPlus</code> is to try each <code>ServerPartT</code> in succession, until one succeeds.</p>
+
+<p>In the example above, the first part is <code>mzero</code>, so it will always fail. The second part will always succeed. That means the third part will never be reachable.</p>
+
+<p>Alas, that means this application will appear to behave exactly like the first application. What we need are some ways to have parts match or fail depending on the contents of the http <code>Request</code>.
hunk ./RouteFilters.lhs 23
-<p>Let's look at some examples.</p>
-
+#include "MonadPlus.lhs"