Genie Vala Generics and Nullable Types

Question

Simple question: In the following generic class, how should the generic type and contained types be defined so that they are nullable? The following will not compile.

class Pair? of G1, G2: Object

    _first:G1?
    _second:G2?

    construct()

        _first = null
        _second = null

    def insert( first:G1, second:G2 )

        _first = first
        _second = second

    def insert_first( value:G1 )

        _first = value

    def insert_second( value:G2 )

        _second = value

    def second():G2

        return _second

Usage:

var pair = new Pair() of string, string
pair = null

Answer 1

Due to the way Vala Generics work, generic parameters are always nullable.

As long as you don't switch on --enable-experimental-non-null class variables are nullable as well, so your code simplifies to:

[indent=4]
class Pair of G1, G2: Object

    _first:G1
    _second:G2

    construct()

        _first = null
        _second = null

    def insert( first:G1, second:G2 )

        _first = first
        _second = second

    def insert_first( value:G1 )

        _first = value

    def insert_second( value:G2 )

        _second = value

    def second():G2

        return _second

init
    var pair = new Pair of string, string
    pair = null

When --enable-experimental-non-null is on, you have to be explicit in the type of the variable. I don't know how to write this in Genie, I tried this, but the compiler does not like it:

init
    pair: Pair? of string, string = new Pair of string, string
    pair = null

In Vala it's no problem:

class Pair<G1,G2>: Object {
    private G1 first;
    private G2 second;

    public Pair () {
        first = null;
        second = null;
    }

    // ...
}

int main () {
    Pair<string, string>? pair = new Pair<string, string> ();
    pair = null;
    return 0;
}