Method and system for creating computer-program-based applications with developer specified look and feel

An system and method for creating applications having look and feel providing dynamically extensible languages for describing application look and feel. Engines are provided for executing programs written in the languages, and methods for extending the languages by creating new language operators are also provided. Methods for joining modules of functionality to new operators allow display of, and interaction with, functionality to be expressed in the extensible languages. The engines and languages permit language operators and functionality modules to be dynamically loaded.