interface Gettable[T] {
    get() T
}

struct Animal[T] {
    metadata T
}

fn (a Animal[T]) get[T]() T {
    return a.metadata
}

// different struct implementing the same interface:
struct Mineral[T] {
    value T
}

fn (m Mineral[T]) get[T]() T {
    return m.value
}

////

fn extract[T](xs []Gettable[T]) []T {
    return xs.map(it.get())
}

fn extract_basic[T](xs Gettable[T]) T {
    return xs.get()
}

fn test_extract() {
    a := Animal[int]{123}
    b := Animal[int]{456}
    c := Mineral[int]{789}

    arr := [Gettable[int](a), Gettable[int](b), Gettable[int](c)]
    assert typeof(arr).name == '[]Gettable[int]'

    x := extract[int](arr)
    assert x == [123, 456, 789]
}

fn test_extract_multiple_instance_types() {
    a := Animal[string]{'123'}
    b := Animal[string]{'456'}
    c := Mineral[string]{'789'}

    arr := [Gettable[string](a), Gettable[string](b), Gettable[string](c)]
    assert typeof(arr).name == '[]Gettable[string]'

    x := extract[string](arr)
    assert x == ['123', '456', '789']
}

fn test_extract_basic() {
    a := Animal[int]{123}
    b := Animal[int]{456}
    c := Mineral[int]{789}

    aa := extract_basic(a)
    bb := extract_basic(b)
    cc := extract_basic(c)
    assert '${aa} | ${bb} | ${cc}' == '123 | 456 | 789'
}

fn test_extract_basic_multiple_instance_types() {
    a := Animal[string]{'123'}
    b := Animal[string]{'456'}
    c := Mineral[string]{'789'}

    aa := extract_basic(a)
    bb := extract_basic(b)
    cc := extract_basic(c)
    assert '${aa} | ${bb} | ${cc}' == '123 | 456 | 789'
}

//////
interface Iterator[T] {
mut:
    next() ?T
}

struct NumberIterator[T] {
    limit T
mut:
    val T
}

fn (mut i NumberIterator[T]) next[T]() ?T {
    if i.val >= i.limit {
        return none
    }
    i.val++
    return i.val
}

fn test_iterator_implementation() {
    mut i := Iterator[int](NumberIterator[int]{
        limit: 10
    })
    for {
        if val := i.next() {
            println(val)
        } else {
            println('iterator is done')
            break
        }
    }
}