First-Class Extents

Adding environments as rst-class values to a language can greatly enhance its expressiveness. But rst-class environments do not mesh well into a lexically scoped language since they rely on identi ers (variable names). By distinguishing variables from identi ers and therefore extents from environments, we present an alternative: rst-class extents. First-class extents are de ned on variables rather than identi ers and are therefore immune to name capturing problems that plague rst-class environments. Then by distinguishing variables from locations and therefore extents from stores, our rst-class extents can coexist with imperative features and still allow tail-recursion to be properly implemented as iteration. To test our claims, we extend Scheme with a collection of features that are essential to rst-class extents, give a denotational semantics for the extension, and demonstrate that it can be fully embedded into Scheme albeit losing tail-recursiveness. Then we show how rst-class extents lead to a way of extending Scheme with object-oriented programming features.