SICP says "Program == Language Evaluator" What does this  mean?
harshavr: The user of the web app has a model and a desired result in mind and somehow needs to communicate this intention to the system. Viewed this way, it is clear that there must be a language in which this communication happens. Of course, we call this the interface. A user interface can be thought of as very high-level,  very domain-specific language.