Constraints: A uniform model for data and control

ABSTRACT

Most programming systems reflect a model of computation which sharply distinguishes between “passive” data objects and “active” program objects (procedures). Furthermore, procedures describe not only a set of computations but the precise flow of control between them. In contrast, a group at MIT is investigating a new paradigm called constraints in which a single kind of object models both data and procedures, and in which the description of procedures minimizes commitment to the order in which computational steps will be executed. A (primitive) constraint is an object with some parts, which correspond to fields of a data structure or input/output parameters of a procedure, and a body which describes how to compute the values of some parts from other parts. The body consists of rules written in an implementation language (Lisp in the MIT systems). Constraints with no body behave like ordinary data structures; constraints with substantial bodies behave more like procedures. However, unlike procedures, which always compute the same set of outputs from the same set of inputs, a constraint may compute in different directions depending on the available data.

REFERENCES

• 1: Hewitt, Carl Actors (MIT Artificial Intelligence Lab memo)
• 2: Sussman, Gerald, and Stallman, Richard Constraints (MIT Artificial Intelligence Lab memo)
• 3: Cannon, Howard Tasteful Flavors (MIT Artificial Intelligence Lab memo)
• 4: Alan Hamilton Borning, Thinglab--a constraint-oriented simulation laboratory., 1979
• 5: Goldberg, Adele, et al. Smalltalk: Dreams and Schemes. (book in preparation at Xerox Palo Alto Research Center.)

INDEX TERMS

Primary Classification:
  G. Mathematics of Computing
  G.1 NUMERICAL ANALYSIS
      G.1.6 Optimization
          Subjects: Constrained optimization

Additional Classification:
  H. Information Systems
  H.2 DATABASE MANAGEMENT
      H.2.1 Logical Design
          Subjects: Data models

General Terms:
Design