I think you hit the nail on the head. I haven't actually used Factor, but I spent 4 years writing and debugging in a concatenative proprietary language, and thi

I think you hit the nail on the head. I haven't actually used Factor, but I spent 4 years writing and debugging in a concatenative proprietary language, and this was my experience too. It was very productive to write in but required great discipline in terms of structure and documentation to be debuggable, and debugging felt very "interactive", where it was easier to catch problems live by watching the data than to solve them analytically. This put a lot of weight on the quality of the debugging tools, which were lacking to the point where it was all about logging values to a file with a tail log on it, and made it sometimes difficult to separate bad data from bad code, which put more weight on the specs being there. But when all the pieces were in place it was the most terse and beautiful code to read.