// callback types
type CBnoret[T] = fn (val T)

type CBnoret2[T] = fn (val T, prev T)

type CBvret[T] = fn (val T) T

type CBvret2[T] = fn (val T, prev T) T

type Callback[T] = CBnoret[T] | CBnoret2[T] | CBvret[T] | CBvret2[T]

interface IObv[T] {
    v    T
    prev T
    cb   []Callback[T]
}

struct Obv[T] {
mut:
    v    T
    prev T
    cb   []Callback[T]
}

fn (o Obv[T]) get() T {
    return o.v
}

fn (o Obv[T]) do_callbacks() T {
    return o.v
}

fn (mut o Obv[T]) set(new_value T) T {
    prev := o.v
    if prev != new_value {
        o.v = new_value
        o.prev = prev
    }

    return o.v
}

fn oo2[T](init T) ?Obv[T] {
    return none
}

fn oo[T](init T) Obv[T] {
    return Obv[T]{
        v: init
    }
}

fn test_main() {
    one := oo(1)
    txt := oo('lala')

    println('txt: ${txt.get()}')
    println('one: ${one.get()}')

    assert txt.get() == 'lala'
    assert one.get() == 1
}