I thought about this idea at a deeper level long time ago and don't remember all the details (and don't have the notes in front of me) but here are two points related to that:
- Programmable debugger would be a massive benefit to compiled langauges like C, and not as much of a benefit for interpretive systems like Python. Especially with the REPL mode of programming in Python (you run something, don't like the results, try something else, then move to next step) is already a pretty effective way to do exploratory prototyping. A programmable gdb would allow REPL like workflow for C.
- As an example, let's say a C program seg-faults at a certain point in code. If you run your C program inside gdb, gdb will stop there and let you do some exploration. That exploration is very tedious if you can't programmatically manipulate your registers and memory areas. In principle, you should be able to write an arbitrarily complex program to bring the seg-faulted state back into a functioning state, and then when the code continues, you don't get the seg-fault again, and you didn't even have to rerun the code.
One use case in which this is massively useful is long running scientific programs. You start a complex scientific simulation based on a C program and it is expected to run for, let's say, 48 hours, but it seg-faults in the 46th hour! If you run it in a programmable gdb you could fix the seg-fault right then and there, and continue the program instead of trial and error of multiple runs, each run taking 46 hours before you know if your fix worked or not.
I think this scenario might require more than just a programmable gdb, e.g., it might require a way for the program to be recompiled and put in memory replacing the older buggy program, before the program has actually finished. But a programmable gdb would be a big part of that system.