Combining Formal Specifications with Test Driven Development

In the context of test driven development, tests specify the behavior of a program before the code that implements it, is actually written. In addition, they are used as main source of documentation in XP projects, together with the program code. However, tests alone describe the properties of a program only in terms of examples and thus are not sufficient to completely describe the behavior of a program. In contrast, formal specifications allow to generalize these example properties to more general properties, which leads to a more complete description of the behavior of a program. Specifications add another main artifact to XP in addition to the already existent ones, i.e. code and tests. The interaction between these three artifacts further improves the quality of both software and documentation. The goal of this paper is to show that it is possible, with appropriate tool support, to combine formal specifications with test driven development without loosing the agility of test driven development.