Program verification in Smalltalk

Fresco is a Smalltalk-based interactive environment supporting the specification and proven development of re-usable software components. These ‘capsules’ are deltas to the inheritance hierarchy, and form a more useful unit of designer-effort than class subhierarchies. Systems are built by composing capsules, which carry both specifications and code. The semantics of capsule composition is elucidated by examining the relationship between ‘type’ and ‘class’. Type-descriptions take the form of model-oriented specifications. The principles discussed here can be applied to other object-oriented languages.