Add -XImpredicativeTypes, and tighten up type-validity checking (cf Trac 2019)

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

diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml
index 26fff9a..3b83551 100644 (file)
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -4501,7 +4501,9 @@ for rank-2 types.
 <sect2 id="impredicative-polymorphism">
 <title>Impredicative polymorphism
 </title>
-<para>GHC supports <emphasis>impredicative polymorphism</emphasis>.  This means
+<para>GHC supports <emphasis>impredicative polymorphism</emphasis>, 
+enabled with <option>-XImpredicativeTypes</option>.  
+This means
 that you can call a polymorphic function at a polymorphic type, and
 parameterise data structures over polymorphic types.  For example:
 <programlisting>