Computer Science School of Computing Clemson University Introduction to Formal Specification Murali Sitaraman Clemson University

School of Computing Clemson University Requirements vs. Specifications  Requirements definition Intended for customers in addition to software developers Informal descriptions are necessary  Specification For use by members of a software development team Formal (mathematical) descriptions are necessary

School of Computing Clemson University Interface Specification  Serves as a contract between component users (clients) and developers (implementers)  Typically describes the demands on users and responsibilities for implementers  Should present the essentials in “user-oriented” terms (abstraction) and hide the inessentials (information hiding)

School of Computing Clemson University Informal Specification Examples  Examples C++ STL Template specifications Java util component specifications  Questions for discussion Do they support information hiding? Do they support abstraction? Can they generalize? Is it possible to make them unambiguous?

School of Computing Clemson University Informal Specifications  Straightforward descriptions Push pushes an object on a stack How much do they help?  Use of metaphors A Queue is like a line at a fast food restaurant Do they generalize?  Use of implementation details Push behaves like AddElement method on Vector Is this appropriate for a user-oriented cover story?

School of Computing Clemson University Informal Specifications  See Bertrand Meyer’s article on Formal Specification in IEEE Computer  Problems with even very carefully designed informal specs Contradiction Noise …

School of Computing Clemson University Formal Interface Specification  Communicates precisely the demands and responsibilities to component users and developers  Allows for independent development of client and implementation components in parallel in a team environment  Minimizes integration costs

School of Computing Clemson University Reasoning Benefits  Formal specifications make it possible to reason about correctness of software formally  Such reasoning may be manual or mechanical (i.e., with automated support)

School of Computing Clemson University Characteristics of Good Specifications  Simple  Clear  Precise  Concise  Implementation-independent  Consistent  Sufficient completeness  Others …

School of Computing Clemson University Languages for Formal Specification  ANNA (and SPARK) for Ada  JML for Java  Larch/C++ for C++  Spec# for C#  …  Eiffel  RESOLVE  …  VDM  Z

School of Computing Clemson University Specification Language Summary  Some specification languages are designed for particular programming languages  Some are general-purpose  Some specification languages are integrated with programming constructs  A few additionally integrate the ability to perform formal mathematical reasoning