can i find out inside a generics function if the t...

# announcements

[rishabhsinghbisht]

can i find out inside a generics function if the type is nullable or not.

[karelpeeters]

Normally you can't find out anything from a generic parameter, but if it's reified you can do null is T.

[tseisel]

Not sure if it is possible, but you can enforce non-null generic parameter :

fun <T : Any> withNonNull(foo: T)

[rishabhsinghbisht]

☹️
require something like if nullable do this else that

[tseisel]

Maybe you could share some code, so that we can suggest you an alternative approach ?

[rishabhsinghbisht]

was trying to create a utility for property delegation from map

private infix fun <Key, T, Value, Return : Value> <http://Key.at|Key.at>(map: MutableMap<Key, Value>): ReadWriteProperty<T, Return?> {
    return object : ReadWriteProperty<T, Return?> {
        override fun setValue(thisRef: T, property: KProperty<*>, value: Return?) {
            if (value != null) { // should be only done if Value is non nullable type
                map[this@at] = value
            }
        }

        override fun getValue(thisRef: T, property: KProperty<*>) = map[this@at] as Return?
    }
}

[rishabhsinghbisht]

usage is something like

var value: String? by PROPERTY_KEY at mutableMap

[karelpeeters]

Then just do what I said?

[rishabhsinghbisht]

even if i do what you said. compiler won’t let me put a nullable value in map as it still doesn’t understand if it is nullable.

[rishabhsinghbisht]

was hoping language has this funcnality

[karelpeeters]

What is "should be only done if Value is non null " supped to mean? Do you mean "Value is a nullable type" or "value is not null"?

[rishabhsinghbisht]

if check should only be done if Value is non nullable type.
(updated on file above)

[karelpeeters]

Ah I see what you mean. Looks like this is indeed not supported, even in new type inference it doesn't work.

[rishabhsinghbisht]

assuming there is a property a using this delegate.

a = null should update value in map if map has nullable type, but if map is non nullable this should just ignore it.

[rishabhsinghbisht]

found workaround for my use-case. 🙂
It got me thinking this problem should happen even for a normal map with non nullable value.
To remove a key I don’t need to do map[key] = null, but instead i can do map.remove(key)