```private fun UserContact?.isValid(): Boolean { ...

# getting-started

[Marko Novakovic]

private fun UserContact?.isValid(): Boolean {
    contract {
        returns(true) implies (this@isValid != null)
    }
    return this != null &&
        userId.isNotBlank() &&
        !email.isNullOrBlank() &&
        !phoneNumber.isNullOrBlank()
}

suspend operator fun invoke(): UserData {
        val userData = getUserData()
        val contact = getUserContact()
        return when {
            contact.isValid() -> userData.fillInWithUserContact(contact)
            else -> userData
        }
    }

compiler is complaining contact inside invoke fillWithUserContact is nullable. shouldn’t contract ensure it’s not treated as nullable?

[hfhbd]

you must use contact.isValid() == true. if contact is null, isValid is not called, and returns null.

[Joffrey]

@hfhbd the isValid() extension is defined on the nullable type, so I don't think what you wrote is correct

[hfhbd]

But when requires a Boolean not a Boolean?. So you need to call == true anyway, and the smart cast works as expected.

@ExperimentalContracts
fun main() {
    val a: String? = ""
    val length = when {
        a.a() -> a.length
        else -> -1
    }
    println(length)
}

@ExperimentalContracts
fun String?.a(): Boolean? {
    contract {
        returns(null) implies (this@a == null)
        returns(true) implies (this@a != null)
    }
    return this != null
}

[Klitos Kyriacou]

@hfhbd the expression contact.isValid() returns a Boolean, not a Boolean? .
In the same way as null.toString() returns a String (with the value "null") and not a String?
So when contact is null , contact.isValid() returns false. It never returns null.

[hfhbd]

Oh, you are right. I did add a useless ?...

fun main() {
    val a: String? = ""
    val length = when {
        a.a() -> a.length
        else -> -1
    }
    println(length)
}

@ExperimentalContracts
fun String?.a(): Boolean {
    contract {
        returns(true) implies (this@a != null)
    }
    return this != null
}

But why does it work for this sample? 🤔

[Joffrey]

Maybe an inference/contract bug? The calling function in the OP is a suspend fun, so that might break inference somewhere