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 de69b60..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>
@@ -5000,9 +5002,13 @@ Type splices are not implemented, and neither are pattern splices or quotations.
    </para></listitem>
 
    <listitem><para>
-   Furthermore, you can only run a function at compile time if it is imported
+   You can only run a function at compile time if it is imported
    from another module <emphasis>that is not part of a mutually-recursive group of modules
-   that includes the module currently being compiled</emphasis>.  For example, when compiling module A,
+   that includes the module currently being compiled</emphasis>.  Furthermore, all of the modules of 
+   the mutually-recursive group must be reachable by non-SOURCE imports from the module where the
+   splice is to be run.</para>
+   <para>
+   For example, when compiling module A,
    you can only run Template Haskell functions imported from B if B does not import A (directly or indirectly).
    The reason should be clear: to run B we must compile and run A, but we are currently type-checking A.
    </para></listitem>
@@ -6374,14 +6380,14 @@ data T = T {-# UNPACK #-} !(Int,Int)
 
       <para>will store the two <literal>Int</literal>s directly in the
       <function>T</function> constructor, by flattening the pair.
-      Multi-level unpacking is also supported:</para>
+      Multi-level unpacking is also supported:
 
 <programlisting>
 data T = T {-# UNPACK #-} !S
 data S = S {-# UNPACK #-} !Int {-# UNPACK #-} !Int
 </programlisting>
 
-      <para>will store two unboxed <literal>Int&num;</literal>s
+      will store two unboxed <literal>Int&num;</literal>s
       directly in the <function>T</function> constructor.  The
       unpacker can see through newtypes, too.</para>
 
@@ -6395,6 +6401,15 @@ data S = S {-# UNPACK #-} !Int {-# UNPACK #-} !Int
       constructor field.</para>
     </sect2>
 
+    <sect2 id="source-pragma">
+      <title>SOURCE pragma</title>
+
+      <indexterm><primary>SOURCE</primary></indexterm>
+     <para>The <literal>{-# SOURCE #-}</literal> pragma is used only in <literal>import</literal> declarations,
+     to break a module loop.  It is described in detail in <xref linkend="mutual-recursion"/>.
+     </para>
+</sect2>
+
 </sect1>
 
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