I had a lot of fun playing with Factor several years ago. The problem, as you pointed out, was that it was indeed difficult to reason about programs. The stack

I had a lot of fun playing with Factor several years ago. The problem, as you pointed out, was that it was indeed difficult to reason about programs. The stack is implicit, so when you glance at code to try to figure out what it does, you have to manipulate the stack in your head as you think about it, effectively carrying out the computation anyway. That became too much for me after a while, and I gave up. Maybe I should pick it up again, though.