A Functional, Functorial Approach to the Model-View-Controller Software Architecture for Command-oriented Programs in ML

We describe a functional, functorial approach to the modelview-controller software architecture for command-oriented programs in ML. In our approach, a controller’s algorithm is apparent; it doesn’t have to be mentally pieced together from a set of event handlers. Furthermore, what would be the state of a model (domain-specific part of the program) in an object-oriented setting becomes arguments to the functions of the controller in our setting. For us, a controller mediates between its view (user-interface) and model. The controller calls its view to get user input, and to display results to the user; it calls its model to do domain-specific work. To increase adaptability, a controller is an ML functor, parameterized by its model and view. One can write controller functors that work simultaneously with both terminaland graphical-based views, although concurrency is needed in order to write graphical-based views that are compatible with such functors. One can also write views that do scripting. Using a shared controller allows one to understand and reason about the program’s command-oriented aspects at a high-level of abstraction by focusing on the controller, as opposed to the multiple and more concrete views. Of particular note is the way we are able to allow computations of the model to be monitored and aborted by the user, via the view. Our approach makes much use of ML’s module system and higher-order, polymorphic functions, and we illustrate it with a case study of a complete application, written in Standard ML, and using Concurrent ML and the eXene X window system toolkit.