1. A system to process a declaratively-specified computer application, the system comprising a processor to provide a functionality of the declaratively-specified computer application, the processor configured to: interpret a structure specification representing a general data model; interpret a behavior specification, wherein the behavior specification uses said general data model represented by said structure specification and specifies a behavior of the application; and process the behavior specified by said behavior specification, wherein to process the behavior, the processor is configured to: identify a behavior component in said behavior specification; and perform a purpose-oriented processing of said behavior component as specified by the behavior specification, wherein the purpose-oriented processing comprises: identifying a behavior component implementation, wherein said behavior component implementation is based on a processing concrete data model, said processing concrete data model is derivable from said structure specification or said general data model; interpreting an application data item using said processing concrete data model; and executing said behavior component implementation to process the application data item using said processing concrete data model and as specified by said behavior component.