[project @ 2003-06-25 08:20:20 by simonpj]

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

diff --git a/ghc/docs/users_guide/using.sgml b/ghc/docs/users_guide/using.sgml
index d3d205a..e1222c0 100644 (file)
--- a/ghc/docs/users_guide/using.sgml
+++ b/ghc/docs/users_guide/using.sgml
@@ -1066,6 +1066,20 @@ f "2"    = 2
          patterns that can fail, eg. <literal>\(x:xs)->...</literal>.
          Normally, these aren't treated as incomplete patterns by
          <option>-fwarn-incomplete-patterns</option>.</para>
+         <para>``Lambda-bound patterns'' includes all places where there is a single pattern,
+           including list comprehensions and do-notation.  In these cases, a pattern-match 
+           failure is quite legitimate, and triggers filtering (list comprehensions) or
+           the monad <literal>fail</literal> operation (monads). For example:
+           <programlisting>
+             f :: [Maybe a] -> [a]
+             f xs = [y | Just y <- xs]
+             </programlisting>
+           Switching on <option>-fwarn-simple-patterns</option> will elicit warnings about
+           these probably-innocent cases, which is why the flag is off by default. </para>
+         <para> The <literal>deriving( Read )</literal> mechanism produces monadic code with
+           pattern matches, so you will also get misleading warnings about the compiler-generated
+           code.  (This is arguably a Bad Thing, but it's awkward to fix.)</para>
+
        </listitem>
       </varlistentry>