And the weird thing is, I realized early in my career that I would actually rather have a runtime error than a compile error. [(some laughs)] Because at that time… now this is way contrary to popular opinion. Everybody wants early error detection. Oh God, not a runtime error, right? But the debugger gives you this ability to start poking and prodding, especially in a more dynamic language, where you can start simulating things, you can back it up… You’ve got your time-machine debuggers like the OCaml one, that can actually save the states and back up.
You’ve got amazing tools at your disposal (in the debugger)… Whereas if the compiler gives you an error that says “expected expression angle-bracket”, you don’t have a “compiler-debugger” that you can shell into…
So, you know, in some sense, your runtime errors are actually kind of nicer.
— An excerpt from one of Steve Yegge’s (long!) talks.
I think there is a real nugget of truth in this. At runtime, you can examine your program’s state, but there is absolutely no way to do that at compile time. Without a debugger, you can’t just look at some nontrivial code and know what the value of x is when there’s an error reading y. (Adding "print x", recompiling, and trying again, would work of course, but that’s just using your compiler as an inefficient debugger!)
Similarly, Strong Typing vs. Strong Testing , essentially argues that some tests can only be made at runtime. (If you read any links on this page, read it, it’s much shorter and to the point).