StepView is an easy-to-use full-screen debugger for the APL*PLUS/PC and APL*PLUS II/386 systems. It displays the currently-executing program in the top half of the screen, with the current line highlighted. Pressing F8 steps execution line-by-line through your program. Stop sets are used to halt your program, so when you stop you're in ordinary immediate execution mode, allowing you to examine local variables and do whatever you usually do when debugging your APL programs. But you can also see exactly where you are in the program. Other function keys allow you to edit the suspended program or copy the current line into the scrolling space for execution.
Although the StepView code is rather old (ca. 1992), it isn't obsolete; I just haven't felt a need to change it. I use it at least once a week and find it an indispensible tool for APL programming. It's very helpful to step through a newly-written program to make sure that variables really are being defined in the way you intended.
Recent versions of the APL*PLUS II/386 system include a more comprehensive built-in debugger, but I find StepView much easier to use and quite adequate for most of my debugging needs. There as some programs, however, that can't be debugged using StepView, including graphics programs and programs that use windowed screen i/o. The documentation has more information.
StepView cannot be used with the APL*PLUS III/Windows system. You must use the debugger built into that system.
Although the read.me file mentions a shareware payment, StepView is now free software. If you want to send me money I won't object, but I'm not holding my breath.