`out` here restricts us from calling add ```fun <T> test1(list: MutableList<out T>) { list.add(l...

# getting-started

[JG]

out here restricts us from calling add

fun <T> test1(list: MutableList<out T>) {
    list.add(list[0])
}

error (from old version of kotlin which gave better errors): Out-projected type 'MutableList<out T>' prohibits the use of 'public abstract fun add(element: E): Boolean defined in kotlin.collections.MutableList'

however, why does in not restrict us from calling removeAt here?

fun <T> test2(list: MutableList<in T>) {
    // public fun removeAt(index: Int): E
    list.removeAt(0)
}

it does work correctly when in is used at declaration site -

class SomeList<in T> {
    public fun removeAt(index: Int): T { TODO() }
}

Type parameter T is declared as 'in' but occurs in 'out' position in type T

[Sam]

The reason add is prevented from being called is because you need to pass an argument, and the compiler doesn't know of any values that inhabit the correct type for that argument. The removeAt function doesn't have any such restriction because it doesn't require any arguments of unknown type.

[Sam]

Its return type is affected, though, so you'll see Any? as the inferred type

[JG]

for use-site in, is Any? the only thing that happens? or is there any case where it is possible to prevent method calls?

[Daniel Pitts]

No way to prevent arbitrary method calls, no.

[Daniel Pitts]

Are you just exploring the options, or are you trying to build something specific?

[JG]

just learning, and while i understood what is explained here, it will still take some time for me to actually get it

[Daniel Pitts]

Fair enough. Most of the time you won't need to explicitly use in or out for type parameters. It's not like in Java where you might need extends and super in generics to make them behave the way you want.