The Tony Hoare quote is about null _references_ which is about aliases to computer memory. (E.g. the dreaded NPE or null pointer exception.) He's not talking ab

The Tony Hoare quote is about null _references_ which is about aliases to computer memory. (E.g. the dreaded NPE or null pointer exception.) He's not talking about _logical_ nulls such as "tri-state booleans" or "missing data". I'm struggling to understand what you mean here. My understanding is that there is no distinction between "logical null" and a "null reference" in terms of the problem we're discussing--as soon as you introduce nulls as a placeholder for values that are not yet initialized you have to deal with the logical implications of null being a member of those types, no? It's been a while since I watched the talk, but scanning the transcript from the talk we're discussing (https://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare https://www.infoq.com/presentations/Null-References-The-Bill...) the way Prof. Hoare talks about them seems to be the same in terms of their logical impact on the type system. I quote: 25:55 One of the things you want is to be able to know in a high level language is that when it is created, all of its data structure is initialised. In this case, a null reference can be used to indicate that the data is missing or not known at this time. In fact, it's the only thing that can be assigned if you have a pointer to a particular type. ... 27:40 This led me to suggest that the null value is a member of every type, and a null check is required on every use of that reference variable, and it may be perhaps a billion dollar mistake. And another thing you wrote which I don't understand: We need nulls in programming languages because they are useful to model real-world (lack of) information. All those other clunky techniques will reinvent the same kinds of "null" programming errors! Isn't that exactly backwards? The entire point Prof. Hoare was trying to make is that using nulls to model the "real-world (lack of) information" causes us to have to contend with the logical problems inherent in making null a member of every type. Additionally, sum types seem to model this in a logically consistent way very well, as other commenters have noted.