Modularizing crosscuts in an e-commerce application in Lisp using HALO

Abstract

Some program concerns cannot be cleanly modularized, and their implementation leads to code that is both hard to understand and maintain. In this paper we consider extending an e-commerce application, written in CLOS, with two of such crosscutting concerns. Though most of the time Common Lisp's macro facilities and CLOS' method combinations can be used to modularize crosscuts, we discuss the use of a more declarative solution when crosscuts depend on the execution history. For this purpose we give an overview of HALO, a novel pointcut language based on logic meta programming and temporal logic, which allows one to reason about program execution and (past) program state.

Symbiosis in logic-based pointcuts over a history of join points

“…In our own work on HALO [9], we have included a symbiosis feature in the language that allows sending base level messages at different times in the pointcut. We have so far mostly studied its applications [9] and implementation using a forward chaining logic [10].…”

“…We have so far mostly studied its applications [9] and implementation using a forward chaining logic [10]. The aim of this paper is to focus on the relationship between HALO and other efforts in integrating symbiosis in logic-based and particularly history-based pointcut languages.…”

Escaping with Future Variables in HALO

“…This application, implemented using the Hunchentoot web application framework, was reported on in earlier work [3] and is used in the experiments in Section 4.2. For explanation, we use the simplified version of this application as shown in Figure 1.…”

“…Secondly, HALO is not purely logic-based: it features an "escape" mechanism which allows methods to be called from the logic pointcuts. Space does not permit us to give a detailed discussion of HALO, we refer to earlier work for a lengthier explanation [3] , but here we illustrate HALO by means of a few advice definitions for the running example. Figure 2 shows an example piece of advice using the two important features mentioned above.…”

“…These were run on the e-commerce application implemented on top of the Hunchentoot web application framework [3]. The pointcuts used in this application can be written in different ways, with or without the use of future variables in escape conditions.…”