I have this code - which can't smart cast in when ...

# getting-started

[Arun Joseph]

I have this code - which can't smart cast in when block, Is this not allowed by design?

sealed interface  UserAccountInfo {
    /** No user info, no token */
    object UnAuthenticated : UserAccountInfo

    /** No token info */
    data class LoggedOut(val userInfo: String) : UserAccountInfo

    /** User info is present, token is expired */
    data class HasExpiredToken(val userInfo: String,
                               val token: String,
                               val userId: String) : UserAccountInfo

    /** User info and token are present, token might be expired */
    data class LoggedIn(val userInfo: String,
                        val token: String,
                        val userId: String): UserAccountInfo
             
}

 
fun main() {
    val userAccountInfo: UserAccountInfo = UserAccountInfo.UnAuthenticated
        when (userAccountInfo) {
            is UserAccountInfo.HasExpiredToken, is UserAccountInfo.LoggedIn -> {
                val token = userAccountInfo.token
                val userInfo = userAccountInfo.userInfo
                val userId = userAccountInfo.userId
            }
            else -> {
                println("Can not refresh token due to lack of authentication data")
            }
        }
}

Gives Unresolved reference: token error.
But if I repeat code, it can work

is UserAccountInfo.HasExpiredToken -> {
                val token = userAccountInfo.token
                val userInfo = userAccountInfo.userInfo
                val userId = userAccountInfo.userId
            }
            is UserAccountInfo.LoggedIn  -> {
                val token = userAccountInfo.token
                val userInfo = userAccountInfo.userInfo
                val userId = userAccountInfo.userId
            }

Is there a way to remove code duplication? https://pl.kotl.in/eT1mSAOfk

[Adam S]

you could create another interface (sealed, if you prefer), e.g.

sealed interface UserAccountInfo {
  sealed interface WithToken { val token: String }
}

and implement WithToken in HasExpiredToken and LoggedIn, and then in your when-statement, check for is UserAccountInfo.WithToken -> {}

[Adam S]

because currently, what should the type of x be?

when (userAccountInfo) {
    is UserAccountInfo.HasExpiredToken, is UserAccountInfo.LoggedIn -> {
        val x: TYPE= userAccountInfo
    }

Maybe one day we will have union types…

val x: UserAccountInfo.HasExpiredToken | UserAccountInfo.LoggedIn= userAccountInfo

But not yet.

[Wout Werkman]

To answer your question; yes. This is by design. There is no duck-typing in Kotlin.
Two types might have the same field or method, but that does not mean that they imply the same behavior.
To imply that 2 types share the same behavior, they need to implement an interface that defines the common behavior, such as @Adam S suggested.

[Wout Werkman]

@Adam S, the purpose of union types is not solve this problem. There is no ticket I am aware of that will introduce this. You can create a ticket (and supply use-cases to improve your odds) if you think this would be a valuable addition to Kotlin.

[Arun Joseph]

Thanks, will try with embedded - WithToken