In other words, it avoids "Tony Hoare's billion dollar mistake" in conflating reference types and optional types. Tony Hoare had a gut feeling that it was goin

In other words, it avoids "Tony Hoare's billion dollar mistake" in conflating reference types and optional types. Tony Hoare had a gut feeling that it was going to cause code messiness when he made all references reference nullable in Algol W (instead of going through and adding extra type rules and tracking for nulls), but it was a very simple change to the compiler's type checker. It was too hard to resist such a simple change that made the language more flexible, even if it introduced footguns. In most languages without ML-like type systems, null is treated as a "bottom type", as if it were a subclass of all classes. This violates the Liskov substitution principle and results in an unsound type system. When I was working on Google's indexing system, I was very excited to hear rumors that Ken Thompson and Rob Pike were working on a new language. When it was unveiled, I was pretty disappointed to learn that it repeated Tony Hoare's billion dollar mistake.