“Seibel: Other than the possibility of implementing it at all, how do you decide whether your interfaces are good? Steele: I usually think about generality and orthogonality. Conformance to accepted ways of doing things. For example, you don't put the divisor before the dividend unless there's a really good reason for doing so because in mathematics we're used to doing it the other way around. So you think about conventional ways of doing things. I've done enough designs that I think about ways I've done it before and whether they were good or bad. I'm also designing relative to some related thing that I've already designed before. So, for example, while looking at the specifications for numeric functions in Java, I'd already done numeric functions for Common Lisp. And I'd documented numeric functions for C. I knew some of the implementation pitfalls and some of the specification pitfalls for those things. I spent a lot of time worrying about edge cases. That's something I learned from Trenchard More and his array theory for APL. His contention was that if you took care of the edge cases then the stuff in the middle usually took care of itself. Well, he didn't say it that way; I guess that's the conclusion I draw from him. To turn it around, you want to design the specification of what's in the middle in such a way that it naturally is also correct on the boundaries, rather than treating boundaries as special cases.”