I'm looking for pointers on how to do something. What I'm trying to do: I want to define a newtype wrapper for database connections with a phantom type to control whether the connection is read-only or read-write. So I have: newtype Conn a = Conn { unConn :: Connection } data ReadOnly = ReadOnly data ReadWrite = ReadWrite -- Simplifying here openConn :: MonadIO m => a -> Conn a query :: (MonadIO m, ToRow r, FromRow s) => Conn a -> Query -> r -> m [s] execute :: (MonadIO m, ToRow r) => Conn a -> Query -> r -> m Int64 But I want to be able to restrict the type a to be either ReadOnly or ReadWrite. Solutions I've come up with so far are: - Don't bother. Later function calls put enough of constraint on the types that it isn't really necessary. Or rather, ReadWrite is necessary, but ReadOnly isn't. - Define some type class that ReadWrite and ReadOnly implement, but don't export the body of the typeclass from the module, preventing other people from implementing it for other types. - Some sort of trickiness with closed type families that I haven't worked out yet. Are their alternatives I haven't considered yet? Thanks. Brian