Quasi-static scoping: sharing variable bindings across multiple lexical scopes

ABSTRACT

Static scoping embodies a strong encapsulation mechanism for hiding the details of program units. Yet, it does not allow the sharing of variable bindings (locations) across independent program units. Facilities such as module and object systems that require cross references of variables therefore must be added as special features. In this paper we present an alternative: quasi-static scoping. Quasi-static scoping is more flexible than static scoping, but has the same encapsulation mechanism. The user can control when and in what scope to resolve a quasi-static variable, i.e., to associate it with a variable binding. To demonstrate its versatility, we add quasi-static scoping to Scheme and show how to build the aforementioned facilities at the user-level. We also show that quasi-static scoping can be implemented efficiently.

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INDEX TERMS

Primary Classification:
  D. Software
  D.3 PROGRAMMING LANGUAGES
      D.3.3 Language Constructs and Features
          Subjects: Procedures, functions, and subroutines

Additional Classification:
  D. Software
  D.2 SOFTWARE ENGINEERING
      D.2.2 Design Tools and Techniques
          Subjects: Modules and interfaces
  D.3 PROGRAMMING LANGUAGES
      D.3.2 Language Classifications

          Nouns: SCHEME

  F. Theory of Computation
  F.3 LOGICS AND MEANINGS OF PROGRAMS
      F.3.2 Semantics of Programming Languages
          Subjects: Denotational semantics

General Terms:
Algorithms, Languages