As bases for object-orientation, both class-based and prototype-based organization have limitations. We argue that roles have significant benefits as a foundation for organizing objects. We further argue that these benefits can be realised most flexibly using logic meta-programming. Additional benefits from this approach are to reduce redundancy and subsume aspects

Holland, Simon

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Open Research OnlineThe Open University’s repository of research publicationsand other research outputsReflective Composition: the declarative composition ofroles to unify objects, roles, and aspects [postersession]Conference or Workshop ItemHow to cite:Holland, Simon (2004). Reflective Composition: the declarative composition of roles to unify objects, roles,and aspects [poster session]. In: Conference on Object Oriented Programming Systems Languages and Applications(OOPSLA 2004) (Vilissides, John and Schmidt, Doug eds.), 24-28 Oct 2004, Vancouver, BC, Canada, ACM Press,pp. 224–225.For guidance on citations see FAQs.c© [not recorded]Version: Accepted ManuscriptLink(s) to article on publisher’s website:http://delivery.acm.org/10.1145/1030000/1028761/p224-holland.pdf?key1=1028761&key2=2089924711&coll=GUIDE&dl=GUIDE&CFID=13972087&CFTOKEN=26001135Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukReflective Composition: the Declarative Compositionof Roles to Unify Objects, Roles, and AspectsSimon HollandDepartment of ComputingThe Open UniversityMilton Keynes  MK7 6AAUnited Kingdom+44 1098 653148s.holland@open.ac.ukABSTRACTAs bases for object-orientation, both class-based andprototype-based organization have limitations. We arguethat roles have significant benefits as a foundation fororganizing objects. We further argue that these benefits canbe realised most flexibly using logic meta-programming.Additional benefits from this approach are to reduceredundancy and subsume aspects.Categories and Subject DescriptorsD.1.5 [Object Oriented Programming].General TermsDesign, Experimentation, Languages, Theory.KeywordsRoles, Logic Meta Programming, Role Models, Composition,Generative Programming, Aspects.1. ROLES AS A BASIS FOR OBJECT-ORIENTATIONThere are practical and philosophical problems withboth classes and prototypes as organising mechanisms forobject-orientation [1-5].  Class-based organisation has well-known limitations in dealing with rapidly evolvingsituations [2,6]; prototype-based organisation, thoughhighly flexible, can be undisciplined without additionalorganising principles [5,7,8]. We argue that an approach toobject orientation based fundamentally on roles has thepotent ia l  for  s ignif icant  benef i ts ;  conceptual,methodological, and practical [1,3,9,10].  Abstract argumentsin favour of role-modeling are well-known, but have to someextent been muted in their force by difficulties inimplementing role-based object mechanisms withoutintroducing new problems [9,11,12]. We consider two suchproblems, and discuss ways of avoiding them, leading to anew approach to organising objects using role-basedprinciples, known as Reflective Composition (RC).Copyright is held by the author.OOPSLA’04, Oct. 24–28, 2004,Vancouver, British Columbia, Canada.ACM 1-58113-833-4/04/0010.The first problem, typically encountered in anyapproach to implementing role-based programming is theproblem of object schizophrenia [9,11].  Informally, thisproblem can be outlined as follows. When modeling adomain using a role-based approach, two kinds of entity areencountered: roles and instantiable objects. The economicalapproach is to model both kinds of entity as objects.However, a problem then arises with object identity, asfollows. When an object plays one or more roles (which mayinteract with each other) then typically (though not always)it is inappropriate from the point of view of domainmodeling for each role to have its own identity, as seen byobjects external to the containing object. To address thisproblem, Reflective Composition uses a sub-object/superobject approach, also used by others such as Bardou [11]. Ineffect, this provides a facility to coalesce an aggregation ofsub-objects into a super-object, after which the sub-objectshave no separate object identity.2. LOGIC META-PROGRAMMING   ANDGENERALISED  COMPOSITIONThe second principal problem addressed byReflective Composition is more general. Loosely speaking,the problem is that, as role-based models become larger, theycan become difficult to organise and re-use. In order to fullyrealise, in a scaleable way, the flexibility and expressivitythat role-based organisation makes possible, ReflectiveComposition uses logic meta-programming [13,14] to factorout the definition of all composition relationships (bothinheritance relationships, in the broadest sense, andaggregation [15]). One way of viewing this is to say thatcomposition relationships are factored out into a separateaspect - though this has nothing to do with the claim that RCunifies roles with aspects – this property arises in a differentway, as described below.In order to allow role composition to be factoredout cleanly, and to facilitate the maximum flexibility andminimal redundancy in the re-use of roles, this aspect i sexpressed by a declarative, reflective, logic meta-programming (LMP) system, which manipulates compositionrelationships between parameterised roles [14,16]. Aparticular LMP program used for this purpose in a givendomain is known as a declarative role composition map orrole map. The resulting role maps may be read as abstracteddescriptions of a role-based model of the domain inquestion. Note that this use of LMP has no connection withthe composition rules of Ossher et al. [17]. The associatedmethod code describing detailed behaviour is typicallyrelatively less complicated than code that has to dealexplicitly with composition relationships. A system ofaliases loosely equivalent to directed resends in Self areused as a mechanism for composing behaviour.3. ROLES,  LMP AND ASPECTSFor fully expressive role-modeling power inarbitrary domains, it is not enough to have the capacity tomodel a single role hierarchy at a time – it is necessary to beable to model role polyarchies – arbitrarily overlappinghierarchies with role nodes or subtrees of roles in common.Declarative role composition maps of the kind noted abovemake directed acyclic graphs of this kind relativelystraightforward to model in a disciplined way. In particular,it is straightforward to control sharing and replication incomposed structures with an arbitrarily fine granularity.  Theability to model role polyarchies directly, coupled with thelogic meta-programming approach to  compositionrelationships give Reflective Composition two interestingproperties.  Firstly, these properties allow code redundancyto be reduced, in principle, to a minimum.  In fact, dependingon the definition of code redundancy used, there does notseem to be any obvious theoretical limit to the removal ofredundancy using this approach. Secondly, because the LMPcontrol of composition relationships allows overlappingrole hierarchies to be effectively switched on and off, thisprovides a relatively simple and straightforward way ofimplementing declaratively quantified aspect orientedprogramming[18]. With this perspective, it becomesreasonable to think of the terms r o l e  and aspect  asinterchangeable for many purposes, without any ‘tyranny ofthe primary decomposition’.4. CONCLUSIONSRoles have both philosophical and practical benefits as afoundation for organizing objects. Logic meta-programmingcan be combined with this approach to abstract outcomposition and inheritance-like relationships. Theresulting framework, known as Reflective Composition,leads to flexibility, a good foundation for re-use, potentiallyreduced redundancy and the subsumption of aspects.Reflective Composition has been implemented in SqueakSmalltalk. Various prototype applications have beenimplemented using this version of RC.5. ACKNOWLEDGMENTSThe work reported here is based on, and aims to continue, thework of the late Henrik Gedenryd on this approach to objectorganization [19]. Thanks to Henrik Gedenryd for numerousdiscussions about his work. Thanks to Kris Gybels andBenedict Heal for very useful questions and observations.Thanks to Michael Jackson, Bashar Nuseibeh, LeonorBarroca, Robin Laney and Patrick Hill for generous anduseful comments.6. REFERENCES[1] Reenskaug, T., Working with Objects: The OORAMSoftware Engineering Method. 1995. Greenwich,Connecticut: Manning Publications.[2] Taivalsaari, A., On the notion of Inheritance. ACMComputing Surveys. 1996. 28(3): p. 438-479.[3] Steimann, F., On the Representation of Roles in ObjectOriented and Conceptual Modelling. Data & KnowledgeEngineering. 2000. 35(1): p. 83-106.[4] Lieberman, H., Using prototypical objects to implementshared behaviour in object-oriented systems. SIGPLANNotices. 1986. 21(11): p. 214-223.[5] Ungar, D. and R.B. Smith, Self, the Power of Simplicity.Lisp and Symbolic Computation. 1991. 4(3): p. 45-55.[6] Scharli, N., et al. Traits: Decomposable Units ofBehaviour. in ECOOP 2003 European Conference onObject-Oriented Programming. 2003. Springer Verlag.[7] Smith, R.B. and D. Ungar, A Simple and UnifyingApproach to Subjective Objects. Theory and Practice ofObject Systems. 1996. 2(3): p. 161-178.[8] Chambers, C., et al., Parents are shared parts of Objects:Inheritance and encapsulation in Self. Lisp andSymbolic Computation. 1991. 4(3): p. 207-222.[9] Kendall, E.A., Role Model Designs and Implementationswith Aspect Oriented Programming. OOPSLA . 1999.[10] Eco, E., The search for the perfect language (Ricerca dellalingua netta cultura europa). 1995. Oxford, Blackwell.[11] Bardou, D. and C. Dony, Split Objects: a disciplined useof delegation within objects. ACM SIGPLAN Notices -Proceedings of 11th  ACM Sigplan Conference onObject-oriented programming, Systems, Languages andApplications. 1996. 31(10): p. 122-137.[12] Gottlob, G., M. Schrefl, and B. Rock, Extending object-oriented systems with roles. ACM Transaction onInformation Systems. 1996. 14(3): p. 268-296.[13] Gybels, K., Using a logic language to express cross-cutting through dynamic joinpoints. Proceedings ofSecond German Workshop on Aspect-Oriented SoftwareDevelopment. Technical Report IAI-TR-2002-1. 2002.[14] Czarnecki, K. and U. Eisenecker, GenerativeProgramming: Methods Techniques and Applications.1999. Addison Wesley: Reading, MA.[15] Lopez, C.V. and W.L. Hursch, Separation of Concerns.College of Computer Science, NorthEastern University,Boston, MA. 1995.[16] Filman, R.E. and D.P. Friedman. Aspect OrientedProgramming is Quantification and Obliviousness. inWorkshop on Advanced Separation of Concerns,OOPSLA. 2000. Minneapolis.[17] Ossher, H., et al., Subject-Orineted Composition Rules.OOPSLA  ACM SIGPLAN. 1995. 30(10): p. 235-250.[18] Kiczales, K. Aspect Oriented Programming. in ECOOP 97Proceedings of European Conference on Object OrientedProgramming. 1997. Springer Verlag.[19] Gedenryd, H., Beyond Inheritance, Aspects and Roles:  Aunified Scheme for Object and Program Composition.Department of Computing, Open University TechnicalReport TR 2002/09. 2002.[20]  Gedenryd, H., Holland S. and Morse, D.R. Meeting theSoftware Engineering Challenges of Interacting withDynamic and Ad-hoc Computing Environments.Department of Computing, Open University TechnicalReport TR 2002/08. 2002.

Reflective Composition: the declarative composition of roles to unify objects, roles, and aspects [poster session]

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Reflective Composition: the declarative composition of roles to unify objects, roles, and aspects [poster session]

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