DrScheme provides a graphical user interface for editing and interactively evaluating Scheme programs on all major graphical platforms (Windows 95/nt, MacOs, Unix/X). The environment is especially well-suited to beginning programmers because it supports a tower of Scheme subsets. Each level corresponds to a particular stage in a typical introductory Scheme course and implements a stringent set of syntactic checks. The environment also pinpoints run-time exceptions in a graphical manner and implements a mostly functional readeval-print loop. DrScheme's most advanced component is a powerful static debugger. It permits programmers to inspect programs for potential safety violations before running them. If the debugger discovers a potential problem, it explains the problemby drawing avalue- owgraph over the program text. The value- ow graphs shows how an inappropriate value mayreach a programoperation and trigger a run-time check. The development of DrScheme in Scheme validated the s...