trying to wrap my head around Contracts. ```class...

# announcements

[Ben Madore]

trying to wrap my head around Contracts.

class Person(val email: String?)
inline class Email(val value: String) 

fun Person.email() {
  return this.email?.let {
    Email(email)
  } 
}

basically an extension method on a class that will null-check a value and return it wrapped in an inline class.

how do i tell the compiler that if the function Person.email() returns non-null then the instance.email property is also not null.

is that possible? am i fundamentally misunderstanding contracts?

[LeoColman]

I think this is possible

[LeoColman]

But I don't know how

[Ruckus]

Setting aside whether that is possible, how would that be in any way useful?

[Ben Madore]

it’s admittedly a contrived example

instance.email()

then at some point later for some reason i need to access the raw value as instance.email and that returns a String? when i know due to checking instance.email() that it’s not null

[Ben Madore]

basically the ability to say:

if some computed property of instance is not null, it implies that property y of instance is also not null

[Kroppeb]

The contracts don't allow you to express this yet

[Dico]

There's very little point in having a class be inline when you make it nullable. It can very much work against you when you box and unbox it over and over. If you want to use inline classes, make sure you understand when they can give you a performance benefit and always view the byte code in intellij to check your assumptions.

[Dico]

For example, last time I checked, comparing inline classes other than unsigned integer variants using == generated instructions to box both operands to perform the comparison with a virtual call (all jvm)

[Dico]

They may have fixed this by now, bottom like is they're experimental, use accordingly

[Ben Madore]

i don’t care about the performance in this case, i only care about the type safety

[Dico]

I think the only use case for inline classes is performance. If you don't care about it, drop the inline modifier.

[Ben Madore]

well, that’s very fair 🙂 though, i think the expressivity of the contract would be exactly the same in this case whether the class is inline or not