Towards situation-oriented programming languages

Abstract

Recently, there have been some attempts towards developing programming languages based on situation theory. These languages employ situationtheoretic constructs with varying degrees of divergence from the ontology of the theory. In this paper, we review three of these programming languages. IntroductionThe development of programming languages based on situation theory [1,6] is a new trend. For this reason, it is worth examining how much these programming languages reflect situation-theoretic concepts and how much they deviate from them. In this paper, we review three approaches [9,4,12] towards programming systems based on situation theory, viz. PROSIT, ASTL, and BABY-SIT, respectively. PKOSITPROSIT (PROgramming in Situation Theory), developed by Nakashima et al. [8,9,10], is tile first situation-theoretic programming language. It is a declarative language in which both programs and data are just sets of declarative elements. This feature makes PROSIT akin to PROLOG, but PROSIT is based on situation theory [1, 2, 6] instead of Horn clauses. The motivation behind +the design of this new language rests on the following features:0 The use of partially specified objects (e.g., situations) and partial information ® Situations as 'first-class citizens' of the theory • Informational constraints ® Self-referential expressions In PROSIT, an infon (a discrete item of information) is represented as a list whose first element is the relation and whose remaining elements are the arguments of the relation: object,).For example, the infon (listening-to John Mary) expresses that the relation listening-to holds between the objects represented by John and Mary, i.e., John is listening to Mary.One can assert infons and make queries about them. Unlike PROLOG, all infons are local to situations. For example, to ~sert the infon mentioned above in situation sitl the following expression is used:Expressions in PROSIT are LISP-style objects (i.e., atoms or lists). Atoms that are numbers or strings are considered Murat Ersan Department of Computer ScienceBrown University Providence, RI 02912-1910, USA me@cs.brown.edu to be constants. Symbols starting with a character other than "*" are parameters. They are used to represent things in the world, such as individuals, situations, and relations. Usually, different parameters correspond to different entities. Parameters can be used in any infon including queries and constraints; their scope is global. A third kind of expression is a variable. Variables axe represented with symbols starting with "*'. They can only occur in queries and constraints. They can stand for any PROSIT expression, yet their scope is local to the constraint or query they participate in.In PROSIT, there exists a tree hierarchy among all situations, where the situation top is at the root of the tree. top is the global situation and the 'owner' of all the other situations generated. One can traverse the 'situation tree' using the predicates in and out. Although it is possible to make queries from any situation abou...

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