Formal specifications are difficult to read.Executable specifications allow to see the behavior of the specified objects and help the domain specialist to detect errors quickly. We present here a method which allows to write axiomatic specifications which can be executed and discuss the limitations in expressive power imposed by the restriction to constructive axioms and how it can be circumvented. The method results from practical efforts to formalize the meaning of object types for Geographic Information Systems. If such data are shared betweenorganisations, differences in the semantics become apparent and formal methods for their definition become necessary. Most formal methods are based on first order languages. Software engineering often uses algebraic methods, but tools practically used for data exchange standard definitions are restricted to signatures and do not capture the behavior of the operations. We present here an algebraic approach using a functional programming language...