[chills]

The article mentions unions vs discriminated unions and then mentions nesting nullable being different than nesting optional, but it doesn't really tie the knot.

Imagine (sorry for pseudocode):

  type Option T = Some T | None
  type Nullable T = T | null

The difference here is that Option "tags" each part of its union, which guarantees that the parts are disjoint. In Nullable, the parts of the union are disjoint only if T is not itself nullable. If T is S | null, Nullable T is S | null | null, which is just S | null (since null and null are not disjoint.

[MereInterest]

I think I'd put it even more on the syntax. Option requires you to explicitly deal with the fact that it is an Option. For nullable types, frequently the syntax doesn't require you to acknowledge that it is nullable at all, and will allow you to continue forward on the assumption that the object doesn't contain null.