Type correctness is pure power, or a waste of time, depending on... perspective! I can't help thinking back to the highly illustrative example of my first enhancement to Facebook's production website as a "bootcamper" in 2009.

I was tasked with adding proper honorifics for Japanese speakers (it's complicated in the general case, but I was just getting as far as "San"). I wrote some straightforward PHP code to implement this feature, but of course it broke in production for some nontrivial percentage of requests. Why? Because the string I was adding the honorific to wasn't always a string. WTF? What is the proper course of action when strings aren't strings? Well, in this case the answer was a run-time type-check guard on the code I had added.

Keith Adams, my esteemed colleague, likened this to "dogs and cats living together", perhaps revealing a fondness for simpler days when Ghostbusters trod the streets. Keith may not have gone as far as lauding OCaml's type system in those dark early days, but I will do so now (and perhaps he would too).

That is not why these languages are popular. Types are your design document in languages with expressive type system. So by design you can make it hard to make logical errors.