+Warning: overlapping instances must be used with care. They
+can give rise to incoherence (ie different instance choices are made
+in different parts of the program) even without <option>-XIncoherentInstances</option>. Consider:
+<programlisting>
+{-# LANGUAGE OverlappingInstances #-}
+module Help where
+
+ class MyShow a where
+ myshow :: a -> String
+
+ instance MyShow a => MyShow [a] where
+ myshow xs = concatMap myshow xs
+
+ showHelp :: MyShow a => [a] -> String
+ showHelp xs = myshow xs
+
+{-# LANGUAGE FlexibleInstances, OverlappingInstances #-}
+module Main where
+ import Help
+
+ data T = MkT
+
+ instance MyShow T where
+ myshow x = "Used generic instance"
+
+ instance MyShow [T] where
+ myshow xs = "Used more specific instance"
+
+ main = do { print (myshow [MkT]); print (showHelp [MkT]) }
+</programlisting>
+In function <literal>showHelp</literal> GHC sees no overlapping
+instances, and so uses the <literal>MyShow [a]</literal> instance
+without complaint. In the call to <literal>myshow</literal> in <literal>main</literal>,
+GHC resolves the <literal>MyShow [T]</literal> constraint using the overlapping
+instance declaration in module <literal>Main</literal>. As a result,
+the program prints
+<programlisting>
+ "Used more specific instance"
+ "Used generic instance"
+</programlisting>
+(An alternative possible behaviour, not currently implemented,
+would be to reject module <literal>Help</literal>
+on the grounds that a later instance declaration might overlap the local one.)
+</para>
+<para>