Document -fwarn-missing-import-lists

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

diff --git a/docs/users_guide/using.xml b/docs/users_guide/using.xml
index 34f769a..cc5045e 100644 (file)
--- a/docs/users_guide/using.xml
+++ b/docs/users_guide/using.xml
@@ -1300,6 +1300,36 @@ f foo = foo { x = 6 }
       </varlistentry>
 
       <varlistentry>
+       <term>
+          <option>-fwarn-missing-import-lists</option>:
+         <indexterm><primary><option>-fwarn-import-lists</option></primary></indexterm>
+         <indexterm><primary>missing import lists, warning</primary></indexterm>
+         <indexterm><primary>import lists, missing</primary></indexterm>
+        </term>
+       <listitem>
+
+         <para>This flag warns if you use an unqualified 
+            <literal>import</literal> declaration
+           that does not explicitly list the entities brought into scope. For 
+           example
+<programlisting>
+module M where
+  import X( f )
+  import Y
+  import qualified Z
+  p x = f x x
+<programlisting>
+          The <option>-fwarn-import-lists</option> flag will warn about the import
+         of <literal>Y</literal> but not <literal>X</literal>
+         If module <literal>Y</literal> is later changed to export (say) <literal>f</literal>,
+          then the reference to <literal>f</literal> in <literal>M</literal> will become
+         ambiguous.  No warning is produced for the import of <literal>Z</literal>
+         because extending <literal>Z</literal>'s exports would be unlikely to produce
+         ambiguity in <literal>M</literal>.
+       </listitem>
+      </varlistentry>
+
+      <varlistentry>
        <term><option>-fwarn-missing-methods</option>:</term>
        <listitem>
          <indexterm><primary><option>-fwarn-missing-methods</option></primary></indexterm>