Beyond static and dynamic scope

Abstract

Traditional treatment of scoping in programming languages considers two opposite semantics: static scoping, where the scope of a binding is a block of program text, and dynamic scoping, where a binding is in effect during the whole reduction of an expression to a value. Static scoping and dynamic scoping are however but two points in the design space of scoping mechanisms. As a result, most proposed language mechanisms that rely on some notion of scoping, such as variable bindings of course, but also more exotic ones like aspects and mixin layers, adopt either one or the other semantics. As it turns out, these two semantics are sometimes too extreme, and a mixture of both is needed. To achieve this, language designers and/or programmers have to resort to ad hoc solutions. We present a general scoping model that simply expresses static and dynamic scoping, and that goes further by allowing fine-grained exploration of the design space of scoping. The model, called scoping strategies, gives precise control over propagation and activation of language mechanisms. While we have already studied it for aspects, we hereby show that the model is not restricted to aspects, by treating in depth its application to the most standard kind of adaptation: variable bindings. We also briefly discuss its application to mixin layers, and program monitoring. We believe that research in programming language constructs can benefit from a more flexible notion of scoping that goes beyond the static/dynamic dichotomy.