SOUL and Smalltalk - Just Married: Evolution of the Interaction Between a Logic and an Object-Oriented Language Towards Symbiosis

The Smalltalk Open Unification Language is a Prolog-like language embedded in the object-oriented language Smalltalk [5]. Over the years, it has been used as a research platform for applying logic programming to a variety of problems in object-oriented software engineering, some examples are: representing domain knowledge explicitly [3]; reasoning about object-oriented design [15,14]; checking and enforcing programming patterns [11]; ; checking architectural conformance [16] and making the crosscuts in Aspect-Oriented Programming more robust [6]. These examples fit in the wider research of Declarative Meta Programming, where SOUL is used as a meta language to reason about Smalltalk code. Recently, we explored a different usage of SOUL in connecting business rules and core application functionality [2], which involves reasoning about Smalltalk objects. We found we had to improve on SOUL’s existing mechanism for interacting with those objects because it was not transparent: it was clear from the SOUL code when rules were invoked and when messages were sent to objects, vice-versa solving queries from methods was rather clumsy. Ideally we would like to achieve a linguistic symbiosis between the two languages: the possibility for programs to call programs written in another language as if they were written in the same [8,13]. Such a transparent interaction would make it easy to selectively change the paradigm parts of an application are written in: if we find that a Smalltalk method is better written as a logic rule we should be able to replace it as such without having to change all messages invoking that method. We will here take a historical approach to describing the SOUL/Smalltalk symbiosis. We would like to provide an insight into our motivation for and approach to achieve the symbiosis by contrasting three distinct stages in its evolution. In a first stage, SOUL was developed as a direct Prolog-derivate with some additional mechanisms for manipulating Smalltalk objects as Prolog values. In a second and third stage we explored alternative mechanisms and a more Smalltalk-fitting syntax for SOUL. Interestingly, when we performed a survey of other combinations of object-oriented and logic programming we found we could