Filtering out non null values from a collection in kotlin

Question

Take a look at this kotlin one liner:

val nonNullArr : List<NonNullType> = nullArray.filter {it != null}

The compiler gives a type error at this line, saying that a list of nullables can't be assigned to a list of non-nulls. But the filter conditional makes sure that the list will only contain non null values. Is there something similar to !! operator that I can use in this situation to make this code compile?

Answer 1

It seems logical to assume that the compiler would take into account the predicate

it != null

and infer the type as

List<NonNullType>

but it does not.
There are 2 solutions:

val nonNullList: List<NonNullType>  = nullableArray.filterNotNull()

or

val nonNullList: List<NonNullType>  = nullableArray.mapNotNull { it }