A modern programming language that makes developers happier.

kotlinlang

Union types taking shape in the IDE <https://kotlinlang.slack.com/archives/C0B9K7EP2/p1589572019120700?thread_ts=1589398414.110200&amp;cid=C0B9K7EP2>

Is it significant that  `help` returns `String?`, not `String`?  I ask, because I noticed that name and hash aren't optional, and the union doesn't appear to allow for null of either type.  Does the compiler not know that returning null from `help` isn't possible?  Is it possible?

Hi <@UJHCLJVUK>, that is just a currently unfinished feature that is pattern matching over unions is not yet exaustive but it will be soon. That is why I used nullable types otherwise is just `String`  .
To your second question yes once that was expressed as pattern matching the compiler knows unless the union includes nullable types in which case the union is of type:

for example that is isomorphic to: `A? | B? | C?`

since a union is a choice null, is expanded as a case and all others are normalized to non nullables

So I understand `A | B | C | null` syntax but including `null` as a type looks weird to me, because nullable types in Kotlin suggest that `null` is a value, not a type

Also how would that work with `String | Int | Double?` or (worse) `String? | Int | Double?`

when a nullable type appears in a union that is the same as if all the types were not null and null was one of the possible values

that is because A? is the same as A | null

All your unions above are isomorphic or the same as `String | Int | Double | null`

a  union can always return null for a typed value that is not the actual value in the union

A? if it did not exist explictly in Kotlin it  would be A | null

I dunno, it just strikes me as super-weird because strictly speaking I think `A | B | C | null` is isomorphic to `A | B | C | Unit`

I am having trouble imagining a case in new code (because union types can only be added to new code) where `null` would be preferable to using `Unit`

the safe usage in kotlin turns it into a singleton type

you can use null for control flow or anything so it  must have a type. The issue is that the type is just the simplification of A | Null