Introduction to Formal Specification Computer Science School of

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

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

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

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

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

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

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

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

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

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

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