I've not debugged Lua but I am familiar with it's integration into C. I imagine debugging Lua+C is no harder than debugging Matlab+C. You load up the REPL in GDB and set your C breakpoints in GDB and your Lua breakpoints in Lua. After control leaves your C function and goes back into the REPL's guts just hit 'continue' to return to the REPL command line. It's more laborious but still straightforward. The only thing you really can't do is step from the high level language to the low one but you can set the proper breakpoint in GDB right before the call so it's more or less the same.<p>-----<p>Edit: sorry not sure if you were talking about Lua or a C-based Scheme, though I imagine it's the same.