Union types taking shape in the IDE <https://kotlinlang.slack.com/archives/C0B9K7EP2/p15895720191207...

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[raulraja]

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

[julian]

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?

[raulraja]

Hi @julian, 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:

[raulraja]

A | B | C | null

[raulraja]

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

[raulraja]

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

[julian]

Cool. Thanks! @raulraja

[Bob Glamm]

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

[Bob Glamm]

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

[raulraja]

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

[raulraja]

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

[raulraja]

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

[raulraja]

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

[raulraja]

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

[Bob Glamm]

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

[Bob Glamm]

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

[raulraja]

null is both a type and a value

[raulraja]

the safe usage in kotlin turns it into a singleton type

[raulraja]

is also isomorfic to unit

[raulraja]

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