derived-instances.md

@@ -7,6 +7,15 @@ Automatic derivation of instances is jolly useful.  Current problems are:
 
 - Automatic derivation is only available for certain classes defined in the Prelude and standard libraries.
 - There is no way to specify how to derive a user-defined class in the general case.
+- There is no way to derive an instance of a class for a data type that is defined elsewhere (in another module).
+- There is no easy way to refer to the definitions that would have been derived when defining an explicit instance.
+- Deriving fails for some Haskell-98 data types. Try
+
+  ```wiki
+  data Rose f a = Fork a (f (Rose f a)) deriving Show
+  ```
+
+  as an example.
 - Even in the absence of user-defined deriving, it should be easy for a 'newtype' decl to inherit some of the instances of its parent type, no matter where the class was defined, but this is currently only available as a ghc extension to haskell'98.