Static Type Inference for a First-Order Declarative Visual Programming Language with Inheritance

Abstract

The early detection of type errors is a well-known benefit of static typing, but until recent years, this benefit usually has come at the cost of requiring the programmer to explicitly declare the type of every object in a program. Since many visual programming languages (VPLs), especially those VPLs intended for end users, are designed to eliminate such programming mechanisms, most VPLs have been implemented with dynamic typing, thereby sacrificing early type error feedback and other benefits of static typing. One potential solution for this dilemma is static type inference, but unfortunately, the types inferred under previous approaches have been notoriously difficult to understand, even for professional programmers. Compounding this problem is the fact that when support for inheritance is added to such type inference systems, explicit type declarations have re-emerged.In this paper, we present a model of types that supports static type inference for a declarative VPL that includes inheritance. Our model addresses the problems presented in the previous paragraph. We present the formal model of our type system, and show that the model is not only sound with respect to type safety, but that it also has sufficient power to support traditional and non-traditional forms of inheritance, and further that it requires the user to understand only a small vocabulary of types, a feature important in addressing the understandability problem in end-user VPLs.