Careful not to mix unions with sum types, though. The key distinction is that the latter are disjunct sets, even if you "sum" together the same type twice, you can always tell which "way" you went.

An example that may show the difference: if you have a language with nullable types, then you basically have a language with union types like String|Null, where the Null type has a single value called `null` and String can not be null.

Now if you pass this around a function that itself may return `null`, then your type coalesces to String|Null still (you still get a nullable string, there is no doubly nullable). This is not true for Maybe/Option whatever you call types, where Some(None) (or Optional.of(Optional.empty())) is different from None only.

Rich Hickey once made a case that sort of became controversial in some FP circles, that the former can sometimes be preferred (e.g. at public API surfaces), as in for a parameter you take a non-nullable String but for returns you return a String|Null. In this case you can have an API-compatible change widening the input parameters' type, or restricting the return type - meanwhile with sum types you would have to do a refactor because Maybe String is not API compatible with String.