> > 4. When would you say that one should use polymorphic variants rather >> than your open datatypes? (I know how to argue in the other direction: >> unique constructors make for better error messages.) >> > > I've wanted such open datatypes several times. One example is a message > bus across an application: some components can yield messages to be > dispatched to all registered components. Messages can hold data, and the > set of possible messages (with the type of their associated data) is > extensible (say, because components can be loaded dynamically, or just to > make the application's architecture more modular). It makes sense to use > an extensible datatype to represent messages. Components can react to > messages with pattern-matching and two components can interact if their > share a common constructor definition. This is simpler than encoding open > datatypes with a "universal" type (with injections/projections). Of > course, one can use the existing "exn" type, but then we don't distinguish > between exceptions and messages at all. >
Isn't this a good use case for polymorphic variants too ? Compared to open datatypes, you could suffer from the weaker type checks though: if two components are supposed to communicate through a constructor, which is mispelled in one of the component, the error would not be noticed by the compiler. my own 2 cent ... philippe. -- Caml-list mailing list. Subscription management and archives: https://sympa-roc.inria.fr/wws/info/caml-list Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
