Null pointers are the billion dollar mistake

@davetchepak "What can C# do that F# cannot?"
NullReferenceException :-)

Educating the imperative gorillas about sum types:

https://chadaustin.me/2015/07/sum-types/

>>2
Amnesiac programming. What if window was already checked for null before close_window got called? Then the check inside close_window would be superfluous, hurting performance with no need.