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kotlinlang

I’m looking at at Kotlin code sample, and I’m seeing `is` used with optional types, which I don’t quite get: `if( myvar is MyObject? )`. Seems to be valid syntax, but I don’t understand what the `?` is adding. Any thoughts?

It’s not really a cast, it’s just an instance check — but … I guess. I’m not sure that’s a value add in this case.

Yeah, fine, I guess that’s why it’s valid syntax. The condition will be true if the variable is null, which lets you avoid `if( myvar == null || myvar is MyObject )` if that’s what you really want.

Although in the code i’m looking at, there’s no reason to be doing that, because the very next thing is to do a myvar?.let or a myvar?.doSomething() on both conditions.

&gt;It’s not really a cast, it’s just an instance check 
... well, in kotlin, it is also a cast. After the check passes, `myvar` can be used as a `MyObject` (or actually a `MyObject?`) without any (extra) cast.

`T?` is the union `T|Null` and asks if the variable is either `T` *or* `Null`, the answer is clear.

It is not true that a `is` performs a cast automatically in-place. It's rather that the compiler inserts a cast after the condition if it boils down to one concrete type where the cast is valid. We can decompose it to something like the following:

```
when (myvar) {
    is MyObject -&gt; {
        val _myvar = myvar as MyObject
        _myvar.doSomething()
    }
    is Null -&gt; {
        val _myvar = null
        doSomethingElse()
    }
}
```

Kotlin does not have real intersection or union types, that is why the `T?` is a hard coded special case in the language. You may want to check out <https://ceylon-lang.org/documentation/1.3/tour/types/> for a JVM language with full support of intersection and union types.

Did you just advertise ceylon in the kotlin slack? :smile:

&gt; It is not true that a `is` performs a cast automatically

... but I don't know any kotlin compiler taht doesn't do the cast automatically after the `is`, so for any reasoning about it, you may assume that is the case.

<@U369WF1KJ> what I did want to clarify is simply that `is` by itself does not perform the cast but rather is sugar. I didn't want to say that you were wrong because from a user's perspective the behavior is exactly the same.