In this paper we demonstrate an approach to model structure and behavior of distributed systems, to map those models to a lightweight execution engine by using a functional programming language and to systematically define and execute tests for these models within the same technology. This is a prerequisite for a smooth integration of model based development into an agile method. The novelty of this paper is the demonstration, how composition and state machine models for distributed asynchronously communicating systems can easily be mapped to a lazy functional language and then using standard testing techniques to define test on those programs. In particular distributed timing aspects and underspecification can be treated accordingly within a functional language, using a certain style of functions.