The direct purpose of this paper – as its title suggests – is to present how the visual evaluator extension is implemented in the GRASP programming system.

The indirect purpose is to provide a tutorial around the design of GRASP, and in particular – around the architecture of its extension mechanism.

Neither GRASP nor its extension mechanisms are, at the moment of writing this paper, final or complete, and we are certain that some details of the solutions described in here will change even before the first release.

What will not change, though, is the set of problems that need to be solved in order to build a system with capabilities similar to those of GRASP. We believe that these problems might be of interest to the Scheme community.