existing-records.md

@@ -309,6 +309,24 @@ foo x@(C2 {}) = x {f1 = 2}
 foo x         = x {~f1, f2 = f2 x + 1}
 ```
 
+### GHC Extension
+
+
+
+Note that in GHC you can work around it like this:
+
+
+```wiki
+foo :: T a -> T Int
+foo x@(C1 {}) = x {f1 = 1}
+foo x@(C2 {}) = x {f1 = 2}
+foo C3{..}    = C3{..}
+```
+
+
+To enable this extension, use `-XRecordWildCards`.
+
+
 ### Comment