Note [Newtype deriving superclasses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+(See also Trac #1220 for an interesting exchange on newtype
+deriving and superclasses.)
+
The 'tys' here come from the partial application in the deriving
clause. The last arg is the new instance type.
| mayDeriveDataTypeable = Nothing
| otherwise = Just why
where
- why = ptext SLIT("You need -fglasgow-exts to derive an instance for this class")
+ why = ptext SLIT("You need -XDeriveDataTypeable to derive an instance for this class")
std_class_via_iso :: Class -> Bool
std_class_via_iso clas -- These standard classes can be derived for a newtype