Hello I have an awaitAll question I am passing it two deferr

Question

Hello I have an awaitAll question I am passing it two deferred values of type `Deferred lt String gt ` `Deferred lt Boolean gt ` and am expecting it to return a List but the result type seems to be of `List lt Comparable lt Boolean amp String gt amp java io Serializable gt ` is there something I am missing here

Jacob · Answer

There s no such thing as a list string boolean

Joffrey · Answer

If you really want to have both values typed properly you can also simply await both of them sequentially It s not 100 equivalent because if the second fails quickly you ll still wait for the first to complete which may be needlessly long but it does the job much better in terms of types ```val s = stringDeferred await val b = booleanDeferred await ```

Joffrey · Answer

If you still want to return them both as one object use a dedicated data class or a `Pair lt String Boolean gt ` instead of a list Lists are meant for homogeneous types of items

Frankie Murillo · Answer

< Jacob> should of said that better meant just a List with those two item types inserted

Frankie Murillo · Answer

< Joffrey> Thank you for the detailed explanation

Jacob · Answer

I m sure you got a list with a string and a boolean but the common interfaces between them is a complicated concatenation of comparable and serializable and that becomes the type of the list

Frankie Murillo · Answer

< Jacob> Correct unfortunately the function signature would seem like it would return the same type as the deferred but it seems that is not that case ```public suspend fun lt T gt awaitAll vararg deferreds Deferred lt T gt List lt T gt ```

Jacob · Answer

It returns the same type as the type parameter T

ephemient · Answer

`T` must be a single type Kotlin tries to infer the narrowest type least upper bound it can which leads to this intersection type

ephemient · Answer

even without reading the specification directly you can see that `String` and `Boolean` are both `Serializable` on JVM `String` is `Comparable lt String gt ` and `Boolean` is `Comparable lt Boolean gt ` so they are both `Comparable lt Boolean amp String gt ` e g comparable to something which is simultaneously both a boolean and a string at the same time no such value exists but that doesn t matter for typechecking those are all of the common supertypes of `String` and `Boolean` other than `Any` `Object` so the least upper bound is `T = Comparable lt Boolean amp String gt amp Serializable`

ephemient · Answer

you can t actually write that type in code <https youtrack jetbrains com issue KT 13108 Denotable union and intersection types> but the compiler can infer it just fine