Author here. Thanks for reading; I was hoping for interesting criticism like this. I don't think that functional programming would have prevented all bugs, or, for that matter, led to better documentation. And I think you're right, there would have been no problem if the system had been documented carefully. I think my point, which I should have made more clearly, is that object oriented languages encourage this kind of programming, leading to systems that are less transparent, while functional languages tend to result in libraries whose mechanisms are less surprising and easier to understand. But surely you're right, it's possible to write confusing code in a functional style as well. And yes, I have worked with large teams on big programs, but that was numerical code, in fortran or C, with old-school scientist/programmers, so neither object oriented nor functional.