Hierarchical Algebraic Nets (HANs ) presented here introduce a new algebraic net class which inherits from a five years experience on algebraic nets corresponding to the work done on the specification formalism CO-OPN [BG 91a]. These works has led to a complete specification formalim supported by a software environnement called SANDS [BFR 93] but who makes obsolete the initial theoretical work done in [BG 91a]. The hierarchical algebraic nets are defined : to answer to the major critics which can be made on CO-OPN and to introduce a specification formalism as a new structured algebraic net class. New features such as the recursive calls of methods or the local association of the data structures specification are some of the major new concepts. The paper essentially defines the syntax and the semantics of the hierarchical algebraic nets in a similar way as it is done for hierachical algebraic specifications which simplify the whole complexity of the formalism. The last part shows how HANs can be used in order to cover the lack of formal semantics to SANDS specifications and what can be the possible evolutions or use of such formalism. For example, HANs have shown their semantical utility through their use to give a formal semantics to a purely Object Oriented evolution of CO-OPN [BB 94a].

Keywords : Higher-level net models, specification, structured specification, semantics, algebraic abstract data types.