For context: This is a simplified example derived from one that the author apparently got from an older discussion started by Peter Van Roy [1] that both of th

For context: This is a simplified example derived from one that the author apparently got from an older discussion started by Peter Van Roy [1] that both of them were involved in (and which I happened to read back in the day). The argument here is about modularity, namely encapsulating internal state within a module without exposing that state to the outside. (For example, so that one can introduce a module-internal auditing mechanism or a cache without altering the module's API.) Peter Van Roy, of course, is one of the principal authors of Oz, so he may have some natural bias, but I've always found the argument persuasive. For testing and validation purposes, one will obviously need to use appropriate techniques (e.g. specifying and validating state invariants). [1] http://lambda-the-ultimate.org/classic/message9361.html http://lambda-the-ultimate.org/classic/message9361.html