That just doesn't happen. People don't check pre-conditions and, frankly, they're right not too: checking all the ways your input might be wrong is often very

That just doesn't happen. People don't check pre-conditions and, frankly, they're right not too: checking all the ways your input might be wrong is often very wordy (much more so than static types), and usually slow. Not to mention that it's not at all trivial to check preconditions; if your program has any kind of duck-typing or dynamic dispatch it's often very hard to check more than the immediately obvious (can it quack?), and that check is often furthermore redundant because you'll use the function. Worst of all, this is extra code that itself can have bugs (and it's code that's likely to see almost no usage in production since you don't make a habit of passing invalid input). Of course, you could check that extra code with extra unit tests... If you want to check preconditions rigorously and pervasively, use a statically typed language (and be a little realistic in what you can check). If you want short code without ceremony, use a dynamic language and don't uglify it with more boilerplate than the worst statically typed language. I've seen that kind of code in ruby, and it's really not a joy. Real life in a dynamic language: people introduce some checks in some high-risk functions, and call it a day. And that's... Just Fine.