Hi:wave:, Can someone tell me why the compile error is going away as soon as uncomment the `student ...

# getting-started

[blue]

Hi👋, Can someone tell me why the compile error is going away as soon as uncomment the
student as Student from the below code? Don’t I have to explicitly assign the cast result to a variable to access the score property?

abstract class Person(val name: String)
class Student(val score: Int, name: String) : Person(name)
class Teacher(name: String) : Person(name)

fun someFun(student: Person) {
//  student as Student
    
    println(student.score)
}

[Joffrey]

Kotlin has flow-sensitive typing, so it can smart cast the student variable to a more precise type (Student) after the as cast even without using a separate variable (simply because it knows the cast would have failed otherwise).

It's the same principle as null checks that smart cast to non-null type inside the if.

[blue]

I’d imagine it will work if I add the below statement instead of the student as Student . But it is not working

assert(student is Student)

[Joffrey]

I guess assert doesn't have a contract, or maybe not one that's precise enough to help the compiler here (EDIT: this is actually because assert uses JVM assertions, which can be disabled). OTOH, the cast with as operator is pretty obvious to the compiler, and it's reliable.

[Joffrey]

Note that require(student is Student) does have a contract that allows this to compile - and it's multiplatform

[blue]

Even check(student is Student) is working

[Joffrey]

Yes, that works too. But keep in mind that they don't have the same semantics. check and require don't throw the same exception (IllegalStateException vs IllegalArgumentException respectively). You should use require if it's input that needs to be checked, and check if you want to verify that some state matches your expectations at some point (e.g. initialization has happened before usage, things like that).