We describe program structuring mechanisms for integrating algebraic, functional and objectoriented programming in a single framework. Our language is a statically typed higher-order language with specifications, structures, types, and values, and with universal and existential abstraction over structures, types, and values. We show that existential types over structures generalize both the necessarily homogeneous type classes of Haskell and the necessarily heterogeneous object classes of object-oriented programming languages such as C++ or Eiffel. Following recent work on ML, we provide separate linguistic mechanisms for reusing specifications and structures. Subtyping is provided in the form of explicit type conversions. The language mechanisms are introduced by examples to emphasize their pragmatic aspects. We compare them with the mechanisms of XML+, Haskell and Eiffel and give a type-theoretic perspective. These mechanisms have been developed within a larger, ongoing prototyping l...