Comment by pjc50

> An API call, reading a file, calling less well-behaved libraries or making some system calls can all result in objects that pass compile-time checks but will cause a runtime exception or unexpected behavior.

Yeah, that is a design flaw that makes this kind of solution less useful than it might be. C# has this problem with "nullable": just because you've marked a type as not nullable doesn't mean some part of the program can't sneak a null in there. Haskell people wouldn't stand for that kind of nonsense.