Doc fix

[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 dee62f4..aafe421 100644 (file)
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -1990,15 +1990,28 @@ main = do
     display (inc (inc counterB))   -- prints "##"
 </programlisting>
 
-At the moment, record update syntax is only supported for Haskell 98 data types,
-so the following function does <emphasis>not</emphasis> work:
-
+Record update syntax is supported for existentials (and GADTs):
 <programlisting>
--- This is invalid; use explicit NewCounter instead for now
 setTag :: Counter a -> a -> Counter a
 setTag obj t = obj{ tag = t }
 </programlisting>
+The rule for record update is this: <emphasis>
+the types of the updated fields may
+mention only the universally-quantified type variables
+of the data constructor.  For GADTs, the field may mention only types
+that appear as a simple type-variable argument in the constructor's result
+type</emphasis>.  For example:
+<programlisting>
+data T a where { T1 { f1::a, f2::(a,b) } :: T a }    -- b is existential
+upd1 t x = t { f1=x }   -- OK:   upd1 :: T a -> b -> T b
+upd2 t x = t { f2=x }   -- BAD   (f2's type mentions b, which is
+                        --        existentially quantified)
 
+data G a b where { G1 { g1::a, g2::c } :: G a [c] }
+upd3 g x = g { g1=x }   -- OK:   upd3 :: G a b -> c -> G c b
+upd4 g x = g { g2=x }   -- BAD (f2's type mentions c, which is not a simple
+                        --      type-variable argument in G1's result type)
+</programlisting>
 </para>
 
 </sect3>