closed-classes.md

 # closed classes
 
 
+
 closed classes are ones for which no more instances may be defined. these can be used to produce better error messages and to refine types.
 
+
 ## proposal
 
 
+```wiki
+
 allow the closed keyword right after class.
 
-```wiki
 class closed ... where
     ...
-```
-
 
 this will mean that even if the class is exported no one may declare more instances for it
 
-## alternate, better proposal
-
-
-specify it in the export list
-
-```wiki
-module Foo(closed Foo(..)) where
 ```
 
-
-this has the advantage that you can create instances in several modules in a private implementation then export them closed in the public interface. it also fits in nicely with the idea of explicit namespaces in  the [ModuleSystem](module-system) and the readonly modifier in the [ReadonlyConstructors](readonly-constructors) proposal.
-
 ## pro
 
+
 - compilers based on jhc's class system can take very good advantage of such declarations when optimizing.
 - This sort of thing is often needed when doing phantom type and type class tricks.
 - this is available in helium in order to increase the accuracy of error messages to good effect.
+- syntax is similar to 'class alias' and 'class overlapping' proposals
 
 ## con
 
+
 - a new language construct