noob here - why don't these match?

# announcements

[dazza5000]

noob here - why don't these match?

[marstran]

Because “registerCallbacks” might call the function with something that is not a CardReaderStatus .

[dazza5000]

But is is looking for Any?

[dazza5000]

Is there a clever way to do this with generics?

[marstran]

It might be a bit more clear if you send a lambda instead. Calling it with { success(it) } would kinda be equivalent, right? But that doesn’t work, because it is of type Any? , not CardReaderStatus

[marstran]

No, I don’t think so. It’s just a type error any way you look at it.

[marstran]

Functions are contravariant in their arguments. This means that the argument type must be of type Any? or a super type of it (which there are none of in this case). So the variance goes the opposite way of what you think here 🙂

[dazza5000]

can one reverse the variance?

[streetsofboston]

Alas, no. An ‘in’ paramater is contravariant at best, invariant at worst

[adk]

The issue is that code inside registerCallbacks might invoke you lambda with an argument of type Any?, but your lambda will only accept an argument of type CardReaderStatus

[dazza5000]

now I am starting to understand how it works a little more - thank you

[dazza5000]

I was dreaming that it would work how I wanted it to 😜

[dazza5000]

but now I see that it might remove some safety mechanisms

[adk]

We've all been there 😉

[marstran]

Took a lot of time for me to wrap my head around contravariance, and why function arguments are contravariant 😛

[streetsofboston]

I look at sub-class/base-class in this way:
It is all about assignability.

A subclass var can be assigned to a base-class var, not vice versa.

When function returns a value of a sub-type, its value can get assigned to a var of a super-type: Co-variant; the assignability ‘points’ in the same direction.

But a function parameter of a super-type gets assigned a value from a provided argument of a base-type. Contra-variant; the assignability ‘points’ in the opposite direction.

[streetsofboston]

interface Function<in A, out R> {
    fun call(a: A): R 
}

[dazza5000]

thank you everyone 🙂