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>