The purpose of functional model of a studied system is to describe what the operations on the system should do. Functional model may contain functional entities of different types. Each type of entity is defined by the functions it contains. Every element in a declarative model are explained originally with the constraints that define their character and the relationships between them. The search program code is kept distinct from the model description. To construct a model in terms of constraints it is necessary to transform its description from the natural language to the language of constraints.