Darren New schrieb: > Joachim Durchholz wrote: >> Actually, in a functional programming language (FPL), you write just >> the postconditions and let the compiler generate the code for you. > > Certainly. And my point is that the postcondition describing "all valid > chess boards reachable from this one" is pretty much going to be as big > as an implementation for generating it, yes?
Yes. It's a classical case where the postcondition and the code that fulfils it are essentially the same. > The postcondition will > still have to contain all the rules of chess in it, for example. At best > you've replaced loops with some sort of universal quanitifier with a > "such that" phrase. Correct. OTOH, isn't that the grail that many people have been searching for: programming by simply declaring the results that they want to see? > Anyway, I expect you could prove you can't do this in the general case. > Otherwise, you could just write a postcondition that asserts the output > of your function is machine code that when run generates the same > outputs as the input string would. I.e., you'd have a compiler that can > write other compilers, generated automatically from a description of the > semantics of the input stream and the semantics of the machine the code > is to run on. I'm pretty sure we're not there yet, and I'm pretty sure > you start running into the limits of computability if you do that. No, FPLs are actually just that: compilable postconditions. Computability issues aren't more or less a factor than with other kinds of compilers: they do limit what you can do, but these limits are loose enough that you can do really useful stuff within them (in particular, express all algorithms). Regards, Jo -- http://mail.python.org/mailman/listinfo/python-list
