Symmetric encapsulated multi-methods to abstract over application structure

ABSTRACT

In object systems, classes take the role of modules, and interfaces consist of methods. Because methods are encapsulated in objects, interfaces in object systems do not allow abstracting over where methods are implemented. This implies that any change to the implementation structure may cause a rippling effect. Sometimes this unduly restricts the scope of software evolution, in particular for methods with multiple parameters where there is no clear owner. We propose a simple scheme where symmetric methods may be defined in the classes of any of their parameters. This allows client code to be oblivious of what class contains a method implementation, and therefore immune against it changing. When combined with multiple dynamic dispatch, this scheme allows for modular extensibility (but not modular type-checking) where a method defined in one class is overridden by a method defined in a class that is not its subtype. In this paper, we illustrate the scheme by extending a core calculus of class-based languages with these symmetric encapsulated multi-methods, and prove the result sound.

REFERENCES

• 1: Allen, Chase, Hallett, Luchangco, Maessen, Ryu, Steele, and Tobin-Hochstadt. The Fortress Language Specification. Version 1.0. Sun Microsystems, 2008.
• 2: Eric Allen , J. J. Hallett , Victor Luchangco , Sukyoung Ryu , Guy L. Steele, Jr., Modular multiple dispatch with multiple inheritance, Proceedings of the 2007 ACM symposium on Applied computing, March 11-15, 2007, Seoul, Korea [doi>10.1145/1244002.1244245]
• 3: Daniel G. Bobrow , Linda G. DeMichiel , Richard P. Gabriel , Sonya E. Keene , Gregor Kiczales , David A. Moon, Common Lisp Object System specification, ACM SIGPLAN Notices, v.23 n.SI, p.1-142, September 1988 [doi>10.1145/885631.885632]
• 4: François Bourdoncle , Stephan Merz, Type checking higher-order polymorphic multi-methods, Proceedings of the 24th ACM SIGPLAN-SIGACT symposium on Principles of programming languages, p.302-315, January 15-17, 1997, Paris, France [doi>10.1145/263699.263743]
• 5: John Boyland , Giuseppe Castagna, Parasitic methods: an implementation of multi-methods for Java, Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications, p.66-76, October 05-09, 1997, Atlanta, Georgia, United States
• 6: Kim Bruce , Luca Cardelli , Giuseppe Castagna , Gary T. Leavens , Benjamin Pierce, On binary methods, Theory and Practice of Object Systems, v.1 n.3, p.221-242, Fall 1995
• 7: Giuseppe Castagna, Covariance and contravariance: conflict without a cause, ACM Transactions on Programming Languages and Systems (TOPLAS), v.17 n.3, p.431-447, May 1995 [doi>10.1145/203095.203096]
• 8: Giuseppe Castagna , Giorgio Ghelli , Giuseppe Longo, A calculus for overloaded functions with subtyping, Information and Computation, v.117 n.1, p.115-135, Feb. 15, 1995 [doi>10.1006/inco.1995.1033]
• 9: Curtis Clifton , Todd Millstein , Gary T. Leavens , Craig Chambers, MultiJava: Design rationale, compiler implementation, and applications, ACM Transactions on Programming Languages and Systems (TOPLAS), v.28 n.3, p.517-575, May 2006 [doi>10.1145/1133651.1133655]
• 10: B. Foote, R. E. Johnson, and J. Noble. Efficient multimethods in a single dispatch language. In ECOOP, pages 337--361, 2005.
• 11: C. Frost and T. Millstein. Modularly typesafe interface dispatch in jpred. In The 2006 International Workshop on Foundations and Developments of Object-Oriented Languages (FOOL/WOOD '06), January 2006.
• 12: W. Harrison, D. Lievens, and T. Walsh. Continuum/j language specification. Technical report, Trinity College Dublin, 2008.
• 13: William Harrison , Harold Ossher, Subject-oriented programming: a critique of pure objects, ACM SIGPLAN Notices, v.28 n.10, p.411-428, Oct. 1, 1993
• 14: Atsushi Igarashi , Benjamin C. Pierce , Philip Wadler, Featherweight Java: a minimal core calculus for Java and GJ, ACM Transactions on Programming Languages and Systems (TOPLAS), v.23 n.3, p.396-450, May 2001 [doi>10.1145/503502.503505]
• 15: D. Lievens and W. Harrison. Symmetric encapsulated multi-methods. Technical report, Trinity College Dublin, 2008.
• 16: Todd Millstein , Mark Reay , Craig Chambers, Relaxed MultiJava: balancing extensibility and modular typechecking, Proceedings of the 18th annual ACM SIGPLAN conference on Object-oriented programing, systems, languages, and applications, October 26-30, 2003, Anaheim, California, USA
• 17: Todd D. Millstein , Craig Chambers, Modular Statically Typed Multimethods, Proceedings of the 13th European Conference on Object-Oriented Programming, p.279-303, June 14-18, 1999
• 18: D. L. Parnas, On the criteria to be used in decomposing systems into modules, Communications of the ACM, v.15 n.12, p.1053-1058, Dec. 1972 [doi>10.1145/361598.361623]
• 19: Peter Pirkelbauer , Yuriy Solodkyy , Bjarne Stroustrup, Open multi-methods for c++, Proceedings of the 6th international conference on Generative programming and component engineering, October 01-03, 2007, Salzburg, Austria [doi>10.1145/1289971.1289993]

INDEX TERMS

Primary Classification:
  D. Software
  D.3 PROGRAMMING LANGUAGES
      D.3.1 Formal Definitions and Theory

Additional Classification:
  D. Software
  D.3 PROGRAMMING LANGUAGES
      D.3.2 Language Classifications
          Subjects: Object-oriented languages
      D.3.3 Language Constructs and Features

General Terms:
Theory

Keywords:
language design, multiple dispatch, software evolution, symmetric methods, type system