ABSTRACT
Simplicity and visibility are two important characteristics of programming languages for novices. Novices start programming with very little idea of the properties of the notional machine implied by the language they are learning. To help them learn these properties, the notional machine should be simple. That is, it should consist of a small number of parts that interact in ways that can be easily understood, possibly by analogy to other mechanisms with which the novice is more familiar. A notional machine is the idealized model of the computer implied by the constructs of the programming language. Visibility is concerned with methods for viewing selected parts and processes of this notional machine in action. We introduce the term "commentary" which is the system's dynamic characterization of the notional machine, expressed in either text or pictures on the user's terminal. We examine the simplicity and visibility of three systems, each designed to provide programming experience to different populations of novices.
