[sacado2]

In theory NULL (or any such sentinel value, such as the infamous NaN is floatin-point arithmetic) makes sense because there are partially defined functions. Division being the most famous example, but most basic operations on lists share that property too.

The problem is the way it's handled in many programming languages, where it can blend so easily with legitimate value and blow the whole program off at execution time.

It's a practical problem, not a theoretical one.

And it's not the same as union types. The division of two numbers always returns a number, and nothing else, it's just that, for some values, it's not defined.

[AnimalMuppet]

I could argue that { number that is defined | number that is not defined } is in fact a union type (or, perhaps more correctly, a sum type).

[bruce343434]

Being pedantic here but "sentinel" means "signalling the end of a sequence" (such as a 0 byte for C strings, or visually when you close an array or a block of code with `}`)