The DrScheme project: An overview

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 pro grammers because it supports a tower of Scheme subsets. Each level correspo nds to a particular stage in a typical introductory Scheme course and imple ments a stringent set of syntactic checks. The environment also pinpoints r un-time exceptions in a graphical manner and implements a mostly functional read-evalprint loop. DrScheme's most advanced component is a powerful static debugger. It permit s programmers to inspect programs for potential safety violations before ru nning them. If the debugger discovers a potential problem, it explains the problem by drawing a value-flow graph over the program text. The value-flow graph shows how an inappropriate value may reach a program operation and t rigger a run-time check. The development of DrScheme in Scheme validated the strengths of Scheme, bu t also revealed several weaknesses. To overcome the latter, the underlying Scheme implementation was extended with a class-based object system, a lang uage of program units, and a sophisticated GUI engine. All of these extensi ons are available to the programmer, who can thus interactively create full y portable, graphical applications.