It's not philosophically wrong to model the problem upfront, it's just very hard to get it right in practice.

You don't literally have to start with one function. If you've solved a similar problem before you may have a good idea of what the code structure is going to be (this is what design patterns are all about). Frameworks may force a structure on you. There are shortcuts. But even then, I've been bitten more then once by the problems that looked awfully similar to what I've done before only to turn out to be very different at the end.